OzVMX Forum
Marque Remarks => Maico => Topic started by: JC on August 19, 2008, 11:57:29 am
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Just following up on some comments on the humble 70s 250 Maico in other posts.
No doubt a VR, Phantom, Pursang, 'snore etc would out-drag it, but they weren't too far off the pace. I do recall that I often got holeshots on my 73 bike in 74 as did other maico pilots in the region, but we weren't national calibre.
In Oct 75, DirtBike printed a copy of the factory modification bulletin recommending porting changes to Ex & Inlet. Ex to be 37mm from top of liner, 58mm wide at top (above transfers) & 52mm wide at bottom. Inlet to be 113mm from top of liner & 52mm wide. Barrels stamped later than June 75 were said to have these specs.
I recently measured 2 barrells from 73 & 74 & both were almost exactly that in heights (didn't check widths), but I don't know if they were stock or not (didn't look all that closely at the time). Transfers were 52mm from top of liner.
That translates to Ex opening about 86.5deg ATDC & transfers opening at about 113deg ATDC for durations of 187deg & 134deg respectively, both of which are rather radical/advanced. Bridged Ex & transfer ports (4 of, plus boost port) are rather large, taking advantage of the benefits of a long-stroke motor. Whether you go on total duration or time-area they're rather hot specs.
So why were they off the pace at the poity end?
Lozza says he gets better performance out of unbridged exhausts & I don't doubt him, but lets stick w the bridged port.
I suggest 1) heavy flywheels consume horsepower to spin & to accelarate, & 2) perhaps the main reason, the inlet duration is only about 160deg (w an 80mm piston skirt which I think was std). Not only that but the sizeable boost port tookup some of the inlet port area, so on both duration & time-area the inlet is very mild (which was typical of euro thinking of the day, tho there were some exceptions)
DirtBike claimed the factory experimented w lighter flywheels on the 250 but never went into prod'n w them.
Seems to me that if you want a faster 250 Maico, start on the inlet area/duration, & if yr real serious, turn some weight off the flywheels.
Anyone tried it?
A friend of mine on a very quick 'snore says he was zapped by a 250 Maico at the Dirttrack nats like he was standing still. No doubt there are some quick ones out there.
Anyone got any other tips that aren't top secret?
Mark, you said in another post that yr flouro-orange one was quick. Do you recall what was done to it?
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John....It was quick for a Maico 250 and a reliable little gem. I'd forgotten about her. It was the only 250 Maico I ever really liked. It was essentially stock except for some slight combustion chamber reshaping to alter the swish area and slightly raise the compression, all of the ports were matched but the port timing and widths weren't changed. I had a Peter Reynolds tapered header pipe, almost the same as that currently on my 350. The ignition was stock points using stock timing and the clutch was from an '80 250. I ran that bike on methanol. I mainly used it for dirt track but I rode her in the occasional motocross and it was a good reliable old bus. I sold it to Barry Collyer who won a lot of races on her until she was destroyed in a garage fire.
(http://img.villagephotos.com/p/2005-8/1062154/[email protected])
My next radial 250 was built from parts using the frame from my spare Maico 350 square barrel, the left over purple anodized wheels and forks from my 440 and a new, almost unused 250 engine I obtained from a bloke who'd bought a new '74 250 and immediately removed the engine and fitted it to an Elstar slider frame. He found it too slow after only one ride and then bought a second hand 440 engine and put the 250 under the bench where it laid for 25 years until I bought it for $500. I basically had Peter Reynolds do the same head mods to it that he'd done to my earlier 250 but left the ports alone, I didn't even blueprint them. I fitted a 38mm Bing from a Husky, and a 490 clutch. Everything else was stock. As I reported elsewhere on this forum we decided to go all out for the '99 DT Nats at Nepean and put the bike on the dyno for three solid days (it was sponsored). With a fresh engine appearing to run well we achieved an underwhelming 17bhp with the stock pipe and carb. After using six pipes and four carb setups and countless jet changes we finally got 27bhp using VP race fuel and the 38mm Bing and a genuine period Wheelsmith pipe I'd had since the day. After all of that fiddling and with race fuel and a bigger carb and trick pipe we finally achieved the factories advertised HP figures. Nonethless, the bike was super quick on the Saturday easily winning the first round over Peter Lee and his 40+hp methanol Elsinore. On Sunday the bike seemed to be a little less perky but Chris Ellis on board her won round two, again from Peter Lee. Unfortunately, in the final round the bike was way down on horsepower and Chris could only manage fourth on a rapidly slowing bike after leading for two laps, relegating him and the bike to second place in the championship. Afterwards we'd discovered a blown base gasket, a pulled barrell stud, and a very cooked piston.
All of this proved to me that Maico bullshitted with horsepower figures for the 250 as we had to really work hard with many an expert twiddling the spanners and flog the living daylights out of the bike to get anything near the performance advertised. A few weeks later we went to Griffith for the NSW titles with the bike on methanol Chris won the over 30 class and 2nd place in the 250 all in class and I came third in the over 40 250 class on her. I recently gave the engine to Les Richters.
(http://img.villagephotos.com/p/2005-8/1062154/OJVMW-santa1.jpg)
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Firko, forget the Maicos, you should have raced a Husky 250 mag they had over 27.5 HP standard. My 250 Mag dyno chart.
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Pete.....The Maico was supposed to have 27hp stock but you and I both know that a Mag 250 Husky will blitz a 250 Maico, all things being equal. I've misplaced my dyno charts, I think they may have got ruined when my garage flooded although I know one sheet survived for sure. Now all I have to do is find the bugger.
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Mark,
How was the dyno driven from yr 250 in those tests? Off the crankshaft sprocket, off the countershaft sprocket, or w the tyre on rollers? (It makes a considerable difference)
Likewise Pete, for yr MAG dyno test?
Many different people put the radial-fin Maico on the dyno back in the day & published them in the magazines of the day, & they regularly made 27hp on the same dyno other 250's made 28-30hp (probably measured at the countershaft sprocket), tho conditions may not have been exactly the same.
That is the key in comparing hp of different engines - they need to be compared on the same dyno under the same conditions if possible or its all-but useless for comparison.
Interestingly DirtBike put a 250 squarebarrel motor on a dyno to test a whole lot of fuel additives back in 72, driving the dyno off the crankshaft sprocket & consistently got 31HP at the crank!! (even w'out the additives) OF course the factory claimed 36HP at the crank as I recall (for both it & the radialfin engine - but I believe the latter had much more advanced porting. Go figure!). Even w the factory bulletin specs of 75 they still claimed, you guessed it, 36HP "on the friendly factory dyno" (to quote DB's facetious wag who wrote it up).
Occasionally someone measured rear-wheel hp (presumably driving the dyno thro the rear wheel on rollers) & the Maico only managed 17hp. The same magazine measured a 73VR at 22.5hp, a 73 Husky (not a Mag) at 17.5hp, a 73 Kaw F11M at almost 21hp (& a 73 400CZ at nearly 24hp, a 450 kaw at only 25hp & 73 MX125 yam at 10.5hp).
That begins to tell more of the story - akin to what yr saying Mark. (Altho, had that Maico not blown a base gasket etc, presumably it would have beaten that worked-over 'snore. I wonder what the 'snore would have produced on the same dyno)
But its the torque figures that tell me the story - the Hus & Kaw have 35% more torque than the Maico & the VR has 40% more. (HP is just a mathematical calculation from torque & rpm.) Interestingly the 450 Kaw has twice the torque & 400CZ nearly twice the torque of the 250 Maico, as you would expect.
Torque is largely related to volumetric efficiency - how much fuel-air mixture you can consistently get into the cylinder to ignite, tho there are other factors like comp ratio. Boyesen notably says, HP is directly proportinal to how much mixture you can get in there, & his whole reedvalve business is built on that. Seems to me he's right.
Which brings me back to the Maico's main limitation being the low duration/time-area on the inlet. That's its biggest drawback powerwise, tho the power-drain of the heavy flywheels doesn't help. (I'm led to believe that the flywheels are the same as the 400 ones, apart from the shorter throw for shorter stroke).
If I still had one, its the inlet time-area I'd start modifying.
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John, we changed the rear tyre for a slick and the readings shown were at the rear wheel. The brand of Dyno is at the top of the sheet.Peter
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Same here, we used a Pirelli Phantom road tyre I had laying around the garage. The figures are rear wheel, similar to Petes Husky. That 17hp Dirt Bike got backs up our results prior to the fuel, carby and pipe changes. As a matter of interest, Cary L'estrange at Condo had Gary Treadwell do a fairly radical port job and achieved around 40 rear wheel HP on methanol with his '73 250. It was rocket fast but extremely fragile. I believe he still owns it and is hanging up in his bar. Peter Lees Elsinore was a quicky as well and reputed to have well over 40 rear wheel horses. Peter lurks this forum and he may be able to back up or debunk the rumours. It was/is damn quick, that's for sure.
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Thanks Peter,
Thats a very good reading then. Was that w the std pipe or w yr aftermarket one?
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In the dyno test the Mag 250 had a aftermarket pipe from Hotrod Husky in the states, it was designed and made by the Maico guru Eric Cook.
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One of the limitations of the strap-it-on-the-rollers method, if yr trying to compare one bike to another, is you can't be entirely sure you haven't got any slippage, & if there is some, how much. Slippage means unmeasured HP. Positive connection via chain elimates that variable, but isn't always an option.
I'm not surprised the Treadwell Maico on methanol got around 40hp. Just looking at the porting design of the std transfers & exhaust, which make good use of the advantages in port area of a long-stroke design (like on all modern 250MX engines), I would think you could get over 30 rearwheel Hp quite reliably w work on the inlet & perhaps flywheel lightening.
Best to keep peak revs down on a longstroke engine tho, to avoid excessive mean piston speed which causes unreliability via ring-flutter etc
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It`s interesting reading things I have tried in the past. I also owned a 250 radial and it was a slug. After a couple of years trying different things I gave up and sold the bike less the broken engine. I had the crank lightened by 10%. Then had spacers made to replace the lost metal in the crank area to maintain a similar crankshaft volume, the result was bearly noticable. The bike accelerated OK but once wound up but lost about 2 bike lenghts out of every corner. A latter 250 of mine now runs the rear transfer which blocks the inlet tract removed and a homemade reed block installed, this bike runs much better. It`s no world beater but at least makes competative power and is much more fun to ride. I`m no expert tuner but my experience has shown the crank lightening not to be useful and efforts put into the inlet port and over-coming the obstruction that the rear transfer creates to be the area that holds the engine back. As has been said the rest of the porting lay-out and timing specs should produce plenty of power but it just doesn`t happen. I tried different pipes, 40mm bing carby which helped, more compression, ignition timing changes, a second cylinder. Finally gave up in frustration. The only thing I didn`t try was methenol. Just my 2 bobs worth.
Another guy in a nearby town had a crank rebuilt with a shorter rod and I don`t know what mods to go with it, he said not much though. I would have swapped him engines any day of the week, again not the most powerful bike but enough to be competetive and fun to ride. He must think so to as he has owned and raced it for around 10 years now, make what you will of that.
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Thats very interesting - shows why a discussion like this is so useful on a forum like this. I'm a firm believer in that famous quote - "men are never more likely to settle a matter rightly than when they are allowed to discuss it freely". I would have tho't the lightened crank would have made some small improvement as the std bike revs so slowly (compared to other 250s), but it seems not in that instance. The std engine is a great engine for mud races!
Not surprised yr mods to the inlet/boost port & reed valve worked quite well. Thats to be expected. I'm not up w the differences rod length makes, off the top of my head. I know it changes port timing but I can't recall which way for longer/shorter rod. I tho't longer rod was generally better, as it also gives more crankcase volume like modern bikes, but no doubt there is an optimum length for every engine. And the std Maico rod is already very long, so it maybe already past optimum length for that engine.
There are 2 other important factors worth considering: direction of transfer ports, & combustion chamber design. ie shape & comp ratio (squish bands etc). A lot of the advance in 2 stroke design for power has been achieved in combustion chamber design over the last couple of decades. The factory bulletin printed by DirtBike in 75 raised C.R. but didn't mention shape/squish-bands. I don't have a head in front of me & I don't recall the shape of std combustion chamber, but it may well be that considerable improvement could be made here. (My vague recollections are of a very simple design.)
And transfer port direction is quite critical.
I recall Brad Lewis once telling me about their experiments on one of Daryll's 250's. One barrell was fast, another was a slug & the only difference they could spot was the direction of the transfers. Slug cylinders did seem to be a problem w the 250 engine. The quality control probably wasn't what it should have been. Thats more than likely. Maybe there were more slug cylinders than 'fast' ones. Even the factory guys were said to try different cylinders to get a good one to start with!!
I was always tempted to adapt a Yam 360/400 top-end & piston to my 250 (same stroke), & still would be if I had one today.
Just on the safe upper rpm limit of this long stroke design, 8700rpm equates to the dreaded 4000ft/min mean-piston-speed safe limit for materials of the day. Beyond that, reliability could be expected to taper off fairly quickly. The std 250 engine readily spun to 8000-8500rpm. Not a lot of margin for error, esp once you start modifying.
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just get a 74 vr250 montesa 8)
or a phantom 8) and save yourselves all the trouble of porting etc ;D
fast on the track and fast standing still ;)
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I dont think there are any good bikes left mister shot. Sounds like you have all of them :P
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heres a gem here brad up your way
http://cgi.ebay.com.au/kawasaki-kx250-1988_W0QQitemZ220272105472QQihZ012QQcategoryZ102690QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
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I don't trust that seller. I reckon he is a bit dodgy ! It's probably a Honda painted green.
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Holeshot,
I agree w you thoroughly - 2 of the best looking bikes in VMX & 2 of the best on the track.
You could add a few others too.
But there's a lot of fun & satisfaction in getting a slower bike 'percolating' up there at the pointy end!
Always loved a challenge.
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then how did the next model 250 engines go like the aw75 76 77
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Paul..With the exception of adding fifth gear which included new cases to house it in '75, the 250 Maico engine was little changed. The '75 model was a bit of a let down after the LTR '74 with its leaky fork seals and dodgy gearbox (in the first batch). By the AW the bike had transformed into quite a lovely bike, albeit one in need of some horsepower injections.
I hope my criticisms of the 250 haven't given the impression that I think they're shitboxes. I had some great trimes on 250 Maicos but their inadequate inlet porting, 4 speed trans and lumpy flywheel placed them in an awkward position on the performance scale. You could get them going. Per Klitland won the Aussie MX title in 1973 and Dave Cunnen and later Chris Ellis won Aussie dirt track titles on the same 250 square barrel in '74/75, way after the squarie had supposedly stopped being competitive. There were many more Maico 250s in the winners circle in the era as well but it was the Maico big bores that set the standard for the others to follow.
Unfortunately, if you are a serious vintage racer and want results but don't want to spend a squillion on porting, PVL ignition and dyno work to get the thing up and going, the Maico 250 isn't for you. On the other hand if you are racing purely for the sheer delight of riding old bikes, the Maico is a ripper. It handles like no other, is absolutely bulletproof once you understand the unique maintenance schedule Maicos demand, and best of all, they have a huge cool factor that can't be matched.
Having said that, if I was an unbiased young bloke of 35 wanting to get into the pre '75 250 class, I'd be looking for a Bultaco Pursang Mk7, OSSA Phantom or Montesa VR. The dear old Spaniards got it 100% right in '74.
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Amen Mark!
Interestingly I just read elsewhere on the web that the big-bores outsold the 250 by at least 3 to 1, often more
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Another statistic that is a little off thread but still interesting is the fact that Maico sold more 490s in 1981 than Honda did in their entire motocross range. What a shame they couldn't keep up the momentum. If things would have gone right, they could quite easily have been as successful as KTM.
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Can't get the challenge of getting the humble 250 up to speed out of my mind!
As I keep pondering the inlet/transfer port areas & durations, I can't help wondering (if you wanted to avoid adapting a reedvalve), how one of these engines would go blocking off the boost port completely, removing the part that intrudes into the inlet tract, & opening up the inlet area considerably & the duration to about 175 degs.
My thinking is that an engine can have too much transfer timing/area for its rev-range. If the transfer charge is too slow-moving, it won't scavenge the cylinder fully/properly. Even w'out the boost port, it seems to me there's still considerable transfer area. Should be plenty for un upper limit of about 8500rpm.
If I had a barrell here I'd measure/calc the time-area of such a mod & compare it to the graphs that are known to work.
I would also seriously look at the combustion chamber, perhaps even (part-)welding it up & machining a good squishband (or two) into it, with a squish clearance of about 1mm on top of the piston (for the outer band). There are some good tips on this elsewhere on the net. (macdizzy? eric gorr??)
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Very funny, 1 fast cylinder and 1 slow 1. What a load of shit, they were both slow. If I had 1 cylinder that went well I would probably still own that bike. The 8700 rpm limit is something I wasn`t aware of but woundn`t doubt, I used to rev the bike pretty hard and high and it was quite unreliable, it had to be ridden that way as there was no power anywhere else.
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Hello D, I wondered if Lewo was you. Thanks for debunking that one.
That is what Brad told me yrs ago. Perceptions I guess, & perceptions are not always accurate. Perhaps he just meant to convey that one barrell was a bit faster than the other. Or less slow!!
I'm still pondering the effect of the size of the transfers & I'm fairly convinced this is part of the problem also. The transfers in the stocker are very high at 52mm. I was very surprised when I first measured it & got a friend to check it.
By comparison the transfer ht on the RM250B/C (which ran the same stroke & 6 transfers) was 56mm & the N 55.5mm, & they were fast. Ex ht is the same on the N & Maico (37mm) & 1mm less on B/C (38mm).
4mm diff on the transfers is a huge difference in both area & duration. As I recall you need for very hi rpm (like roadracers), but as we've seen, the Maico can't achieve anything that.
If somebody could measure the width of the transfers & boost port (& give me some idea of their angle), I could calc the time-area & find out for sure.
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I'll pull a barrel this week.
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Thanks James.
Can you also measure the size of the crank please - ie dia & thickness of each half. I want to compare it to some of the other 250s of the day. I have a feeling it'll be more than 10% heavier.
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Hi kawboy, probably quite true 1 a bit better than the other. Like firko I adore old Maicos and anyone who can make a 250 run hard has almost god status with me. Over the weekend the idea of a Yamaha cylinder mounted on top was a great idea I couldn`t get out of my head. I still have the complete bottom end, unfortunately nothing else. Talking off transfers, I also thought the transfer timing might be too radical for the rest of the engine. As a test the transfers were lowered with devcon and the angle squared up more like a newer style cylinder. The result was a total waste of time and effort, again not a noticeable difference. Could the rear transfer be done without? I think so as at 1 time I had the hole in the piston welded up, couldn`t tell any difference with this change. If an original looking cylinder is important and I was taking on this challenge again I think the rear transfer could be somehow blocked off and an inlet port shape made to a more common shape and duration then maybe the motor would respond,a lot of work if it was a failure though. I really liked the bike but it simply was not a fun bike to ride as good as the suspension and handling was, the motor let it down. To me this is an interesting thread and look forward to any other info, cheers.
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Darryl and John.....Maybe I've missed something in the thread but why on earth would either of you want to put a Yamaha top end on a Maico? ??? The Yamaha bottom part already has a clutch that behaves itself, a five speed greabox,a bulletproof big end bearing, better transfer ports and is lighter. It's all right doing that Nutty Professor stuff if it improves the breed but that swap doesn't make much sense to moi :o. Instead....fitting the whole Yamaha MX250 engine to a Maico like my old mate Peter Rowlands legendary Yamaico special makes far more sense that fitting a nice Yamaha top end to such an archaic bottom end as the 250 Maico. Just enjoy the slow old 250 Maico for what it is...A good reliable (if somewhat slow) old bus that is a pleasure to ride and cool to show off at Classic Dirt.
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Thanks James.
Can you also measure the size of the crank please - ie dia & thickness of each half. I want to compare it to some of the other 250s of the day. I have a feeling it'll be more than 10% heavier.
That will take a while, how about I get the specs from a friend who has one blown apart. My bottom end is together now.
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That'd be fine James.
You can do it approximately w the crank still in the engine if yr careful by pushing the 'tail' of some vernier calipers down between the crankhalves to the bottom of the cases & read it to the top of the flywheels. That'd be w'in a couple of mm which would do for these purposes as I suspect they significantly larger dia than other 250s.
My guess is the Maico flywheels are as much as 130mm dia w 25mm wide halves. I just checked Suz, Hon, Yam & Kaw 250 flywheels (by the above method) & theyr'e all about 110mm w 25mm halves. Mon & Ossa 250s are under 100mm dia w about 22mm halves & Bul about 96x25 (tho they generally run larger ignition flywheels than the japs). For what its worth I think its the spanish who got the flywheel weight right in the sweet spot & is one of the keys to their great performance.
Its not just the weight of the flywheels; the dia is even more critical. It takes a good deal more power to overcome the angular momentum of a kg at 130dia than it does a kg at 100 or 110 dia. So even if you take a kg or 2 off the maico flywheels, it makes a difference where you take it from. I'd be turning down the OD if possible.
The Hp taken to spin the extra weight at say 8000rpm may only be .5 - 1 hp at best (I'm only guessing), but its the power taken to accelerate the flywheels (coming out of corners) that concerns me. As I recall thats where the 250 maico really lost ground.
James, if you (or anybody else) could measure the Ex & In ports overall widths & bridge widths also (as well as trans), I'll do time-area calcs on them all. My guess is Ex is somewhere in the ballpark, Trans are way too much & In is way way too little. The In wont be easy to measure w its unique shape.
Mark, good question!
Reasons are many & varied for fitting 360 (not 250) Yam top end. Cos its a challenge & sense of achievement, cos its something different & I always had a hankering for something different, cos Yam barrels look like Ossa phantom barrells & I love the look of Phantoms, cos its another alternative to going the 326 Maico route (got some more questions on that too which I'll post in the other thread in due course), cos I'm not convinced the 4sp box is much/any disadvantge on many/most tracks, cos they both have the same stroke,... & for me (perhaps most of all) its unfinished business.
I first wanted to do it back in 75 when my 73 250 was hardly competitive, but I never had the resources. Back then I always thot the 400 Maico was the quitessentential MX bike. A 360 Yam top end would give me the next best thing & the shorter stroke would give me a revvier bike w less of the famed/dreaded vibrations of the long-stroke 400 Maico. I couldn't remotely afford it then, but I've wanted to do it ever since. Also, its a bit like climing Mt Everest - cos its there!
Daryl, yes I enjoy the thread too. For me this sort of thing is the forum at its best & most usefull - people pooling their knowledge/experience/expertise for the benefit of all. There's an old proverb, "As iron sharpens iron, so one man sharpens another". Its so true. I'm no Eric Gorr or the like, but I've long had a keen interest in the engineering details of MX (VMX) bikes, esp things like 2 stroke porting & pipe design, frame design, steering geometry etc. I just love to find out why a thing works or doesn't work. I may be thick, but I can see no reason why we can't find out why the 250 maico underperforms (& fix it) if we apply good sound engineering principles.
Thanks heaps for yr input. Its vital to finding the probs & solutions. You've tried more things, w sounder thinking to get to the route of the problem (& gotten closer) than anybody I know to get the 250 maico engine working properly.
I keep finding out more interesting/relevant things which I'll post in due course. (Too crook at the moment) Will respond more to yr tho'ts above too w.r.t yr work on the transfers. I think you were on the right track, but went 1step forward & 2 steps back. Also some tho'ts on what I've already found/measured about fitting 360 Yam top end. Another option if one wanted to keep it a 250, is a Kaw F11 top end.
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Ok Gents, lets roll again.
I'll leave the top-end conversions to another topic/thread which I'll initiate in due course (hopefully soon)
This 250 maico engine is unlike any other I've seen & got the specs for over the years, & that includes almost all VMX 250s except obscure things like AJS/Greeves.
As they say, "the devil is in the details' & I think its true here.
If we compare duration specs alone you might say its not much diff to some others:
Maico Ex 187 Tr 134 In 160 (these could be higher cos I only guessed the rod length &
Phantom Ex 183 Tr 133 In 168 I think I guessed short)
VR/V75 Ex 180/188 Tr 126 In 157
KTM Ex 187-88 Tr 126-27 In 156-57
But... the devil is in the detail. You need long durations for:
1) hi RPM
2) small ports &/or only 2 or 3 transfers - to compensate for small area
3) short stroke engine - cos available port area is less than for longstroke engine.
eg Phantom is short stroke (60mm) & has only 2 transfers, so its tr dur'n in particular is long.
VR has three trans (tho the 3rd is quite small) & shorter stroke than maico (64 vs 70mm)
KTM has plenty of transfers but is short stroke (62mm) so port area is not so large
Maico has none of those 3 things above - it can't rev hi safely (before it exceeds 4000ft/min mean piston speed), it has 4 transfers & a boost port, & all the benefits in port area of a long stroke design - except on the inlet. The considerable reduction in area caused by the boost port casting is not compensated for by longer duration. And the transf duration is huge for an engine w large trans port area - suitable for 11,000-12,000rpm according to recognized rules of thumb!!
So how much area do we lose in the lnlet for the boostport casting? I scaled off a pic I have of a 250 Squarebarrell liner - apx area of port w'out the casting intrusion is 10sq cm; apx area w intrusion is 6sq cm. Now throat area of 36mm carb is just over 10sq cm. We have strangulation.
Inlet port area at the liner should be 10-15% greater than that at carb, not 40% less!!!
The radialfin engine's inlet maybe be a little better but its a similar design &...you get the drift.
KTM also has boost port but it does not interfere w the inlet at all. So does the VR but its inlet design is much better than the maicos. Interestingly I believe the 400 Maico doesn't either - just a big beautiful clear inlet & big beautiful power. Also interestingly, Bultaco went to a boostport intruding into the inlet on the 370 pursang (Mk9/10?) but went back to no boost port on Mk11. Wonder why? They can be made to work, as on the 250 Buls (Mk9 onwards) & VB montesa 250/360, but both have much smaller intrusions into the inlet tract than the maico.
So, theory # 1 - this engine is strangled at the inlet. This is incontrovertible it seems to me.
Solutions: a) open up the area around the boost port casting (I wouldn't reduce the width of the vertical bridge) to at least the area of the carb throat if possible, b) increase duration (lower port, cut piston skirt).
Now onto the transfers. I noticed when DirtBike did its stage-tuning of the 250 squarebarrell they said, "The boost port looks as tho it can easily be widened some more to let the engine breathe better, right? WRONG! (emphasis theirs) Fuel/air velocity will be lowered & low end torque will disappear as a result"
So what did they do on the radialfin engine? Add another two transfers! (& raise port height & duration as well I think).
Theory #2 - the meagre volume of charge allowed in thro the restricted inlet moves too slowly thro the huge transfers & cylinder to scavenge the cylinder properly. Consequently each stroke burns a lot of pre-burnt mixture, not a (mostly) fresh charge & so is 'asmatic'.
Possible solutions: open up the inlet & close down the transfers/boost. On this theory Darryl you should have been on the right track lowering the ht of the transfers, however in squaring them up as well, you effectively opened up the area again (cos the critical area is measured perpendicular to the direction of flow. ie more square-on, more area). One step forward & one step back - the 2 effects tend to cancell ea other out.
Plausible theory, but I'm not convinced. Here's why. (Remember, the devil is in the details.) I noticed 2 diff mags independantly (in 2 diff years - 73 & 74/5) found the power output super sensitive to the stinger/muffler arrangement (tested/confirmed on dyno). In 73 the bike came std w a very small dia & very long stinger but the dealers cut them off & sold them w mufflers. DirtBike could only get 26.5 HP w the muffler on, but when they used a std pipe w the looong stinger they got 5Hp more (29) at 7000rpm & 4HP more (30.5) at 7750rpm. And in 74/5 Cycle World tested a bike that came standard w an Xdusor muffler (remember them - weird shape/design which, as I recall, was said to offer good silencing w very low backpressure) in states w low loise-level regulations & could only get 23HP, but when fitted w a (not so quiet) skyway, they got just over 27HP. Almost unbelievable difference (& indepentantly verified to a reasonable degree). Why?
Beats me... till I read Gordon Jennings & A Graham Bell again. I'm still far from sure, but I think I have some idea. The stinger affects/regulates the bleed-down pressure & consequently temp inside the chamber (& probably all the way back into the cylinder). AS far as I understand it, that makes the pulses stronger & faster in there. (Sound waves are temp sensitive). It seems to me that: 1) this would effectively make the pipe shorter in tuned length, (the same efffect should be achievable w a different designed pipe), 2) this engine is more critically dependant (than other engines) on strong/correct exhaust pulses to generate decent power & 3) this engine works best w some backpressure. Which leads me to...
Theory #3 which in part is exactly the opposite of #2 - ie that this engine is so strangled by the inlet that it depends far more on exhaust pressure pulses working thro long exh & transfer durations to create hi suction in the crankcase to draw a half-decent charge thro the pitiful inlet rectrictions, but in the process, on account of the looong transfer duration, (& if the pressure pulses/timing isn't spot-on) the fresh charge has enough time (in this engines rev range - it might not at 11,000rpm!) to continue on out the exhaust wasting all that potential power it has worked so hard to get into the cylinder. This would also explain why it seems to work better w some back-pressure.
I think theory #3 is the most likely. (#1 is a given as far as I'm concerned)
On this theory, I think you were heading in the right direction Daryll, reducing the transfer duration & squaring them up. Shorter dur'n gives less time to escape out exh & squaring them up slows down the charge by crashing ea side into the other & diffusing their kinetic energy (according to Graham Bell, which you probably read too). However... shortening the transfer dur'n also reduces the suction in the crankcase cos the pressure pulse to create it is also shorter. Again, one step forward & 1 step backwards.
So what's the solution?? Well, I'm not Einstein, or Dr Gordon Blair but it doesn't take a rhodes scholar or rocket scientist to work out that step 1 must be opening up the inlet considerably, I would suggest in both area & duration (but go 1 step & a little at a time). I think the main benefit gained by the reedvalve is from this. But I'm still working on a sol'n for those who don't want to go that way esp if they're still illegal for pre75.
Next, I think both exh & transfer dur'ns are too long. (I like the specs of the RMB/C engine which has the same 70mm longstroke design & ample transfer area, but I doubt we'd get the transfers down that low w'out MAJOR work which I'm trying to avoid) The simplest way to reduce Ex & Trans (& in this case probably the best) is to lower the barrel by maching the base (which will also increase inlet dur'n), but I don't know how far that can be done cos I don't know how close the top ring is to the top of the liner at TDC - perhaps only 1mm. (Can somebody measure it pls). Almost everybody involved in porting 2strokes says evn 0.5-1mm can make considerable diff (but it doesn't always) Shim the head to obtain 1-1.5mm squishband clearence.
After that, I'd perhaps try lowering the ht of the boost port (only) w devcon, partly cos its much easier to do than the side transfers, & patly cos its closer in the scavenge loop to the exhaust (thereby reducing the potetial for the charge to escape out the exh w the shorter dur'n)). Keep the same angle initially, but perhaps try redirecting it as another step.
This is perhaps the appropriate time to design a new pipe to suit the new porting. (Alternately, a new pipe design could be tried after you fix the inlet. A diff pipe design is always worth a try at almost any step).
The last 2 suggested steps require a lot more work: reshape the combustion chamber to have 2stage squish-band & skim the flywheels.
I still think the flywheels could do w lightening. More power & better spread is great (if we're successful), but yr still going to have difficulty exploding out of corners if the engine revs too slowly. I'll say more on this in another post.
If I were doing this, I wouldn't be removing the boost port to open up the inlet (unless yr running a reedvalve - then I'd just mill thro the alloy at the back of the boost port, down thro the inlet port). Its there & I'd leave it there - there's too much work involved in removing it & its irreversible. I'm trying to avoid both. I think one should be able to get sufficient inlet area & duration by widening & lowering the inlet tract, perhaps w some triming of the semicircular outer casing of the boost port that protrudes into the inlet tract. Be careful not to widen the ports too far. Again, a little at a time.
But REMEMBER Gents, these are suggestions - NOT guarantees. You'll have to check every detail on the actual barrell yrself for ea step. Remember, the devil is in the detail!
And PLEASE, only do one step at a time to see if its progress or not. There may only be small progress w ea step, but DON"T launch in & do several steps at once. You won't know what worked or didn't work.
Don't shoot me if they don't work. I've stuck my neck out making these suggestions according to how I understand it, but pls don't chop it off. By all means, add yr thots tho - pool our understanding/insights. Thats what we need - informed discussion. My theories could be entirely wrong. Shoot down my theories/solutions if you want, just don't shoot me.
Remember, what you do on yr bike is YOUR responsibility. I don't even have a bike to try it on (haven't owned a maico for 30yrs - much as I'd like one). Maybe I'm flying completely blind.
Now my head hurts. I'm going back to bed to try & shake the flu - & recoupe some of the sleep I lost lying awake last night thinking this out.
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Would I be so bold as to suggest that you seem to have a bit of spare time on your hands John? ;D
I recently got a similar lengthy diagnosis when I asked Jens Olsen to build my TM400 engine when it arrives. He produced his little exercise book that went into a similar detail as to why the TM400 is basically a dunger and a lengthy excursion into his theories on fixing its shortcomings. I'm a Philistine when it comes to this stuff but I still find it interesting. Good one John (and Jens).
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Hi Kawboy,
Great work and Firko you are BOLD.
But in your obvious haste, you seem to have forgotten the intimate relationship between torque/horsepower and gearing [both primary and secondary] in optimising on track performance [and out of pity I won't mention chassis/traction optimisation].
Those additional considerations shouldn't add much to the difficulty of your analysis - only by an exponential of 100 or so!
Back to it. I look forward to your revised conclusions. When complete I will be putting your name forward to be updated to MaicoMan - you will be a mere Kawboy no longer!!
VMX42
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ummm kawboy can we go through that again.. ;D
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Im not reading that!!!
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mmmmmmmmmm :o
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Wow, my head hurts just reading the above. Simply things first, I agree the crank seems to heavy but the crank is standard in my AW and it runs fine and doesn`t feel like it needs to be lightened. As you have said these things run almost no ignition flywheel weight, you could machine it but I don`t think the amount of work is justified by the results.
Because of the dykes ring the base of the cylinder cannot be machined very much, from memory .5 mm which had no noticable effect on power.
Maybe the pipe is the easiest place to find power, I wasn`t aware of the stinger/ silencer information. Maybe push the stinger down inside the rear cone and mount the silencer on the back of this, it doesn`t have to start at the finish of the rear cone and hopefully the missing 5 hp will come out of hiding. I no longer have a test mule either to try this on but it would have to easier than cylinder mods and able to be reversed should it not work. Would be interesting if anyone has tried this. Does anyone have the exact stinger dimensions? Good luck !
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Mark, yes yr right - when yr crook you can't do much else. Also I enjoy a challenge like this. Its right up my alley cos its good to dust off the 'gingerbeer-skills' occasionally. Also, my philosphy on life is that problems are just challenges waiting for a solution. (Pity I couldn't do that w my health)
VMX - I assume owners have worked on (2ndary) gearing changes (primary ones are a lot more difficult) & chasis/traction issues to optimise the performance of their bike for their weight/riding/tracks etc. In fact, the flywheel wt is one such issue. But so does everyone else. What I'm trying to fix is what was widely regarded as a dud engine. You can optimise all the other things but if the engine isn't making enough power... yr still behind the 8-ball.
My 1st loves in bikes are Buls & Kaws, w Mon, ossa & maico close behind, probably in that order. My 1st race bikes were bul, then kaw, then maico & they've pretty much remained 'in the blood' ever since. "Kawboy" was just a pseudonym that sounded good at the time.
090 - you don't have to. But if you have a 4sp 250 radialfin, it could be worth it.
Lewo, yes I wondered if the barrel could only be dropped a little. I know the dykes ring is only 0.5mm from top edge of piston (I still have my old 73 piston) but I didn't know if the piston is at the top of the liner at TDC. I'd just remove the base gasket & use liquid gasket, then shim head for 1-1.5mm squish-band clearence.
Flywheel weight perhaps is more a matter of rider preference/style. Taking weight off the ign flywheel would be all-but useless, cos its small dia has min effect on rotational momentum. I'm not really pushing this issue (I think the pipe & inlet tract much more critical), but some facts are worth considering. With 130dia flywheels their wt is about 11lb as opposed to about 8lb for 110dia ones (common to many 250s of the day). That extra 3lb is spinning out there between 110 & 130 dia, & 3lbs spinning at that dia is about .6ft/lbs torque (only about .75hp at max rpm). But its the quicker responsiveness that I'd like for getting out of corners. For what its worth, skimming the flywheels 3mm removes about 1lb as I recall, but I don't know if the balancing holes are far enough away from the edge to allow that (or more). Lot of work tho.
Re: the pipe. The maico factory chief was at the DirtBike test back in 73 & it was him who told them to run the std stinger, claiming "All the old concepts in silencing do not apply to this pipe & this motor". But I'm not conviced he's right now, simply because you can achieve the same effect as a small long stinger by using steeper baffle cone. Because of the long transf dur'n this engine appears to need a strong +ve return pressure. If you look at the std 73/4 pipe, the included angle of the baffle cone appears little more than for the diffuser, thereby returning a broad but fairly weak pulse. No doubt maico did that to achieve a broader powerband neccessitated by the contraints of 4sp box & heavy flywheels. But it deosn't solve the prob created by the long dur'n on the transf. I noticed that he 5sp & (esp) AW pipes appear quite different in this area - which is probably why yr happy w yr AW, D.
Yes, the long stinger can be mounted down inside the baffle cone. Its a good move & thats exactly what I did in 74. (Perhaps that's why I always tho't my bike was fast enough) It also has the great benefit of silencing the bike enormously. My bike sounded like a Bul frontera, if you know how quiet they were. Have to be careful w small dia long stingers tho cos they raise the combustion/piston temp & can fry/seize the piston.
Apart from a small long stinger or a steeper baffle cone, I would think this engine would respond to a tapered-header pipe (cos they conserve the strength of the pressure pulses more) & perhaps a 2-stage baffle (should give broad & strong return pulse). Me worth trying on a MOTA program (or similar).
D, I checked the effect of shorter rod, & it does reduce timing duration - just what this engine needs esp on tranfs, IMHO. Thats probably why yr friends bike worked better w this mod. Lots of work tho, & I would think you could only reduce it by 5mm or so before you reduce the thickness of the bottom flange on the barrel too much when you mill the corresponding amount from there. Any more & you'd weaken it too much to secure the barrel to the cases properly. I would have tho't 5mm would make little diff & to do more you'd have to beef up the (top of) the flange before you machined the bottom. Huge job!
So, for what its worth, my current revised suggestions are (in order of preference):
1) Open up the inlet area & duration
2) Use liquid gasket in lieu of gasket at cyl base & shim head for correct clearance
3) Mod pipe w internal stinger of original siz, or (preferably) redesign pipe
4) Optional - skim the flywheels (ie if yr deadly serious & can justify the amount of work)
5) Optional - modify ht/direction of rear boost port (again, if you can justify the amount of work)
I think 1 & 3 will make the most diff.
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:) Terrific thread fellas. I'd been hanging out to try a 326 conversion ever since it cropped up on the Forum some time ago. Now it all begins to fall into place - the 326 is quick not only due to the increased displacement but because it seems that the 400 barrel design ( porting ) is so far ahead of the 250.
I'd been headscratching over how the various port timings could be made to work using the 400 pot and the shorter stroke 250 crank. Firko mentioned machining 10 mil off the 400 base flange which is a fair chunk. Any other options ?
I'd be really interested in being able to get in touch with anyone who has done these conversions ( especially if there's ' nine different ways ' like skinning the cat ) to kick it around in more detail.
Great job you've done on why the standard 250 doesn't go so well, kawboy. Will be looking forward to Chapter three.
Cheers, T250.
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Hmmm a few to many assumptions made here, mainly not so much the duration makes a difference but where they occur in relation to each other.
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This thread has been a revelation. Even if Johns theories are a little off kilter (and I'm not suggesting they are), it's opened up some interesting dialogue on a bike that is often dismissed as being very ordinary. Despite their weaknesses, many folks have indeed got the Maico 250 working well enough to win championships on them but in stock trim it's a very big ask. Isn't this better than 12 pages of Suzuki footpegs? ;)
*At the pub last night I was reminded by Chris Ellis of the square barrel Maico he won a NSW title on in the mid 70s. The bike had been imported in the very early seventies by Blair Harley for Per Klitland to ride and was reputedly an ex Adolf Weil factory bike. It was a revvy rocketship that was quicker in every department to his '74 440. Unfortunately the bike was destroyed by subsequent owner after he blew a gearbox and stripped the bike and sold the parts. We both remembered that the flywheels were drilled like swiss cheese with the holes plugged up with cork and araldite. It also has a 1mm longer DT1 Yamaha rod. I owned the crank and barrel at one stage after I found them in a box of stuff I bought from a bloke in Blacktown. Sadly I didn't appreciate their importance and I sold 'em on to someone else. Maybe the speed of this bike was essentially down to the radically lightened crank? ??? I don't recall the barrel having anything radical done to it.
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No disrespect to Suzuki fans ( there are three of them standing in my shed ), but WAY better than 12 pages of Suzuki footpegs Firko.
Even John is saying he's still working through it, and it's not over yet, but I reckon the way he started from scratch and compared the Maico 250 breathing apparatus to a number of successful contemporary 250's was a touch of brilliance. I was loaned a Maico 250 for a ride at CD4 by a generous guy from Queensland who had been telling me for years that they're not very quick. To a non owner, that seemed to be the most frequent comment ever heard about them.
After thirty years or more of perplexing criticism, maybe we're finally beginning to understand why they're not very Quick.
Technical info of this quality is of enormous interest and value to an absolute beginner like myself. I would also like to see a detailed discussion on Flywheel lightening, as I believe it to be another black art which should be considered ( and usually isn't ) very carefully in conjunction with porting, pipe design, gearing, and all other factors including the most common bike usage / track.
Thanks fellas. It's been said before, but this Forum really is one of the best around. Cheers, T250.
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Lozza,
Hope you read pages 1&2 also.
I'm floating ideas as to what might be the cause of this engine's generally poor performance & what might be done about it.
Feel free to elaborate on yr comments & add to the discussion. In fact Pls do. As I said earlier, I very much believe the saying, "men are never more likely to settle a matter rightly than when they are allowed/encouraged to discuss it freely". Thats my motto in many ways.
I know I'm 'flying blind' at times. Having read Eric Gorr again, I'm not certain all I said about the effects of a small dia long stinger is correct. (altho he's writing about modern engine/pipe design, I believe). And I don't even have a barrell here to work from, let alone an engine/bike/pipe. (I did check out a couple of barrels recently tho, even if all too briefly.) In my own thinking I work w individual port opening/closing timing, blowdown etc (I think the blowdown is a little short on this engine) Its just easier in posts like this to quote dur'ns.
What I'm trying to do is to piece together all the relevant pieces of info that I've accumulated on this engine/bike & the experience of others to get the overall picture to explain an underperforming engine/design. Or at least, stimulate discussion to that end. Its been an interesting ride.
But its very much a work in progress - hopefully for the benefit of those who want to make their 250 radialfin faster.
But pls add yr insights - we'll all be the better for it
I'd love to put the specs into a MOTA program or the like, but I don't have that facility.
Mark, very much lightened flywheels would give the IMPRESSION of being a much faster engine, just as heavy flywheels give the impression of being slower, but thats not necessarily the case. I would think there must have been some porting work as well if it was so fast, & I don't doubt either squarebarrel or radialfin engine can be made to boogie.
Personally I think the inlet 'strangulation'/bottleneck is indisputable - the rest is largely still theories.
Whatever anyone has to contibute, pls do so
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John..That 250 square barrel Maico not only felt faster than Chris's 440, it was a bike length quicker in a drag race and had had an equal punch out of corners. Per Klitland won the '72 Australian 250 title at Christmas Hills on it and the next owner David Cuneen (son of 60s speedway midget legend Bryan) won a number of dirt track titles with it and the next owner, Chris Ellis, who you met at CD5 in the Kevlar Kompound won Victorian dirt track and long track titles as well as a second in the Aussie DT Championship. It was a tempermental bike with it suffering the occasional transmision and clutch glitch. I raced a '73 250 radial at the time and when I occasionally rode the squarie it felt 100cc bigger than mine.
By saying that there didn't appear to be any radical porting didn't mean it was a stocker, in fact it was far from it. What I meant was that there wasn't anything radically different from other ported Maicos I'd seen. The combustion chamber had also been reshaped and it ran a 36mm Amal MK2 on methanol and had a Wheelsmith rev pipe and later on a Gary Treadwell fat pipe, one of the first seen in the country. I still own the Treadwell pipe but it doesn't work on any of my bikes which points to it being a part of a unique combination that made the 250 so damn fast.
Blair Harley told me a few years ago that he found the bike sittling up the back of the shop at the Maico factory in '71 when he was over there checking out the new radials. It seems that it was an old works bike ridden by Adolf Weil as it had his numbers and perculiar handlebar bend. Blair was fuzzy on the details but he thinks the factory sent it to OZ with his first shipment of radials. I don't think that's right as Per would have been riding the latest radial at Christmas Hills if that was the case. I think it would have come in with the last squarie shipment 6 months earlier. Whatever its travel arrangements, it was a well used looking thing with a yellow painted alloy tank and alloy airbox which also pointed it towards being factory bike. Later Chris painted the tank up in US flag Stars'n'Stripes for a bit of flat track showbiz pizazz.
I'd love that bike today, especially with it's works history. Unfortunately all that remains to my knowledge is the pipe, tank and frame. The other bits could be anywhere.
I'll be catching up with Per Klitland over the next few weeks and I'll work on him to try and recall some of the details of the engine.
BELOW: The great Adolf Weil on a square barrel 250 Maico, perhaps the bike in question
(http://img.villagephotos.com/p/2005-8/1062154/adolf-weil.jpg)
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Fascinating, Mark! I'd love someone to find the rest of that bike too. Shame the media of the day didn't make anything of having a factory/works Maico in the country like they did w Per's OW years later.
Didn't mean to imply it wasn't fast - just that it would take more than lightened flywheels to make it so fast. Also backs up DirtBikes assertion that the factory experimented w lightened flywheels on the 4speeds. I have a sneaking suspicion, from something DirtBike said in the Stagetuning the 250 Squarebarrel article in 72, that its transfers were 2mm lower than the radial's. If my analysis of the radial is correct, it would have been better if they'd kept them at that lower height.
Here's another story of a fast 250 (AW). In 76 I attended the Oz titles at Tivoli & Bevan Blackers (if my memory is correct) from WA, whom I'd never heard of before that, creamed the best of Oz's best (they were all there) in 2 motos that day by a country mile, except that a few laps from the end of one moto his front wheel started de-spoking. As he slowed it took a fair while before anyone caught him, but alas it became so loose he couldn't finish (as I recall). Such a crying shame for one who had driven so far, I nearly cried. I never remember who won the 250 title that day - just that Blackers so clearly deserved it. Trevor Flood won the 500/Unlimited title that day an an AW400 so Maico got some much-deserved glory. But it should have been doubled.
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Gents,
A few tho'ts on the 4sp box. I'm not convinced its much of a disadvantage on most tracks (unless there's a loong fast straight or two). Perhaps its more psychological. The ratio spread (1st gear ratio divided by top gear ratio) of the 4sp Maico is 2:1 w a fairly good close spread of ratios. In fact the 'snores 1st 4 ratios are very close the the maicos.
The ratio spread of most of the close ratio 5sp bikes is about 2.2 or 2.25:1. But what do most 5sp bikes do? Start in 2nd gear & use only the top 4 gears on most MX tracks. ie effectively operate as a 4sp on the track.
Keep a larger countershaft sprocket or two in the toolbox & you shouldn't be at too much of a disadvantage.
W.r.t the power, I'm still of the opinion the main limitations are the pipe & inlet, w the trans specs being a bit radical too. Which to fix 1st is a bit of a chicken & egg situation. The pipe/stinger/baffle cone has to be fixed to make reasonable/decent power, it seems to me. But you've still got serious strangulation at the inlet. With the size of the ex & trans ports, it also seems to me you could get some serious power out of the engine if you fix both. But remember the max limit of 8750rpm for reliability.
To give some idea of the effect of removing the basegasket, IF that safely lowered the barrell by 1mm, you would reduce the ex duration by 4degs (to 183deg) & the trans by 5degs (to 129deg), but increase the inlet dur'n by 4deg (to 164deg). Worth considering. Thats getting closer to RM250B specs.
Combustion is apparently 'sweetest' w a 1.0 - 1.5mm squishband. Its worth checking/adjusting. (It will of course be diff from standard if you lower the barrell)
If I had the port widths, I could calc the time-area values & be more conclusive. Until then, my hunch is as above.
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PS I'm lead to believe the 73-75/6 cylinders were much the same inside so if anyone has a cylinder in that range on the bench & can measure the width of the ports, that would suffice. I'll calc time-areas from that.
Its not hard to do if you use a pair of dividers (from a compass set). Just open them up to the width of the port inside the cylinder, then pull them out & measure the distance.
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Just dug out the lightened crankshaft and measured for anyone interested. Just for a reference as the amount removed was I think 10%. This amount could safely be increased as this crank made no noticeable difference. Maybe if more was removed throttle response may change but I don`t believe lightening the crank will in any way increase power or acceleration. The weight of maybe a 0.5 kg of crank mass against a 100 kg bike and 80 kg rider is not going to change anything, also nothing changes in relation to filling the cylinder with extra air and fuel.I would only consider this if rebuilding the crank as pressing apart a good crank to machine it in my opinion be a waste of time and effort. Below are the measurements I ended up with.
Each crank wheel is 22.7 mm wide,
Diameter is 121.8 mm.
Good luck, Lewo.
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That would be 100% correct. VHM charge many many $$$ for their 'heavy' cranks for RS125 road racers, doesn't alter output makes abig difference to lap times.
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D,
Thanks for measuring the crank. This is perhaps a good time to have a bit of discussion on flywheel-effect, as T250K suggested above (or on another thread).
It certainly deserves consideration independant from the power production of the engine, but there is some overlap.
On those dim'ns, compared to a typical Jap bike (except YZa/b which have hollow flywheels!) the Maico crank is 13% heavier, & 11% larger dia - which is just as significant. Why?
Flywheels are storage devices (the mechanical equivalent of a battery) that operate via rotational momentum. Momentum stores energy which can either be absorbed or given up for short periods (just like a battery does electrically).
Now linear momentum is just the mass times the velocity. In the case of flywheels & angular momentum, that velocity is proportional to the dia of the flywheel as well as rpm. And its the extra dia where the Maico carries the extra weight so its a two-fold multiplyer effect.
So, as a rough approximation, the flywheel-effect of the maico is 1.11x1.13 (= 1.25) that of a typical jap 250 at the same rpm. ie. 25% greater, & from memory thats about how it feels too. Its considerable.
It may seem like 0.5 - 1kg extra flywheel weight is insignificant compared to a 100kg bike + 80kg rider, but once you get past optimum, its the-straw-that-broke-the-camels-back scenario. An extra 0.5-1kg spinning at 120mm dia @ 8000rpm is 0.5-1Kg moving at 50m/sec, which is a considerable force to be overcome.
But what effect do flywheels have in practice? Frankly I laugh every time I hear/read: "put a flywheel weight on to give it some torque" or "lighten the flywheel to give it some power" as if the flywheel can produce torque/horspower itself. Torque & horsepower (which are directly proprtional to ea other via a mathematical formula) are produced only by how much fuel-air mixture you combust, at what rpm, & how efficiently you harness the energy released.
If you do dyno runs w the same spec engine but w diff flywheels weights, stabalizing the rpm after you've raised it for ea reading, the diff in torque/power will be all-but negligible & probably not measurable.
However.... lets not forget the storage/momentum effect. When you are lugging an engine the flywheels (if sufficient weight, like the Maico) can give-up energy for a short period, & that is why they're so popular on trials bikes. Conversely when you accelerate the engine up its rev-range the flywheels absorb energy that something else must supply, ie the engine. (in much the same way you have put energy into a child's spinning-top/gyrscope to get it up to speed).
In that situation you have to first overcome the momentum-effect which holds it back. Plus you are raising the flywheels to a higher energy state (higher rpm) which requires energy input (just like the battery/spinning-top). That's the situation where too-heavy flywheels can & do hold back acceleration.
The test for this would not be a dyno run, but a roll-on acceleration comparo (w other effects neutralised)
It seems to me its made worse on the maico by it not having a very fat midrange to overcome that extra flywheel-effect when accelarating up the rev-range. On the other hand, the YZ(a/b)s also lose out cos of too-little flywheel effect, losing any potential gains in useless wheel-spin.
Obviously, if yr riding style is to use the breadth of the powerband coming out of corners rolling the power on smoothly from lower in the rpm range (as mine is) you are going to be held back more by too-heavy flywheels than if yr style is to keep the rpm up in the corners & row the gear lever. (Unless its a slippery track surface.)
On the other hand, heavier flywheels can be a distinct advantage for the average rider in the drag-race to the 1st corner, in that the momentum-effect tends to keep the engine spinning in its higher-rev/higher-power range rather than falling below it.
So it seems to me there is an optimum flywheel weight for every engine/track-condition/riding-style combination. Both too-heavy & too-light can affect acceleration to some degree. They also affect ease/difficulty of riding. To my mind/style its the spanish who got it in the sweet-spot best. Yam went too far one way, & Maico too far the other.
T250K also asked about the flywheel-effect of clutch weight & rear-wheel weight. It seems to me these are minimal, probably not even noticeable to the average rider, mainly on account of the significant reduction in rotational velocity due to drastic gearing reductions.
Having said that, I recall one incident when Marty Smith's mechanic replaced the rear tire on his RC125. He came back in complaining that the bike felt as slow as a tractor. They found that the new tire was 2-3lbs heavier & Marty felt it. OF course the effect would be multiplied by higher unsprung weight giving less traction as well.
But yr perfectly right, DL in saying its a lot of work reducing flywheel wt, & I wouldn't consider it either unless the bike needed a new rod/big-end. And then I would be skimming it off the OD of the crank where possible, where it has most effect.
You are also perfectly right in saying it does nothing about the intake limitations of the std Maico. Same about the pipe/stinger, & the transfer ports/timing. Those need to take MUCH higher priority.
Apologies for another lengthy post, but thinking things thro from 1st principles explains/resolves most problems.
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Again your making to many assumptions John and the maths are miles off.As above heavy cranks have advantages over light cranks.
http://www.vhm.nl/VHM/indexcrankmod.htm
http://en.wikipedia.org/wiki/Moment_of_inertia
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Yes I know Loz. I nearly said as much, but I was trying to simplify the maths into something understandable for the average VMXer, avoiding the integral calculus along the way, but giving some approximate measure of its effect.
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Loz, Having had a chance to look at yr suggested web-sites, I maybe thick, but I really don't see what the VHM site is supposed to prove/disprove. Unless I've missed something, all it really says (thats relevant) is that some bikes can benefit from extra flywheel wt. I've never doubted that, & tho't that was plain in my post.
And I have to question the relevance of the example of an RS125 roadracer to a 73 Maico 250 MX, which is obviously what's being discussed here. Have you ever raced/ridden that model Maico? Just about anybody who has notices the heavy flywheels & feels its slow.
Regarding the other website, I doubt if 1 in 1000 VMXers could care. I'm well aware that the maths for angular momentum (moment of inertia) is much more complex than linear momentum & if you want an error tolerance of +/- 0.1% or even 1% then be my guest - do the complex mathematics or use a computer analysis. For the purposes of my discussion a ball-park figure is all that was required.
Likewise the assumptions. Its hardy uncommon in engineering analysis.
"The moment of inertia of an object about a given axis describes how difficult it is to change its angular motion about that axis. For example, consider two discs (A and B) of the same mass. Disc A has a larger radius than disc B. Assuming that there is uniform thickness and mass distribution, it requires more effort to accelerate disc A (change its angular velocity) because its mass is distributed further from its axis of rotation: mass that is further out from that axis must, for a given angular velocity, move more quickly than mass closer in. In this case, disc A has a larger moment of inertia than disc B.
The moment of inertia of an object can change if its shape changes. A figure skater who begins a spin with arms outstretched provides a striking example. By pulling in her arms, she reduces her moment of inertia, causing her to spin faster (by the conservation of [[angular momentum]])."
That is taken from the wikipedia website, & that is precisely what I sought to describe in my earlier post.
With all due respect, I'm not interested in one-up-manship. I'm just trying to help those VMXers who have such a bike understand its shortcomings & what maybe done to overcome them. ie get the information out there in laymens terms; not be coy about it.
Its a bike/engine I'm familair with & interested in (& have considerable info on), having raced one for 2yrs back in the day, probably spending 100's hrs on it. I don't pretend to be infallible, but I trust others can benefit from the analysis.
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(http://i96.photobucket.com/albums/l187/mx250a/icon_lol.gif)(http://i96.photobucket.com/albums/l187/mx250a/icon_lol.gif)(http://i96.photobucket.com/albums/l187/mx250a/icon_lol.gif)(http://i96.photobucket.com/albums/l187/mx250a/icon_lol.gif)(http://i96.photobucket.com/albums/l187/mx250a/icon_lol.gif)(http://i96.photobucket.com/albums/l187/mx250a/icon_lol.gif)(http://i96.photobucket.com/albums/l187/mx250a/icon_lol.gif)
....although you are taking all the fun outa VMX. What fun would there be if we only rode and raced and didn't do a little reverse engineering correcting the mistakes of the past. ;) 8) ;D
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I would love the time to type a 10 page summary on all the things that could be wrong with this engine.What I am trying to highlight is oversimplications and lots of theory without field/dyno tests is basicaly doesn't help anyone.As problems could be all,part or none of what's been outlined.Also engineers might do things we might think is 'all wrong' but in fact we don't have a full and proper understanding of engine dynamics. As with the heavy crank, which I know that 99% of 125 racers no little about or a very dissmissive of...a heavy crank improving lap times...???? However it does exactly that and priced accordingly.As a very wise GP level tuner told me "an assumption is an error, yet to be uncovered" ;D
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JC, still need port widths and heights etc.? I just FOUND my 76 250 engine-it is more assembled than I thought too.
I will give a Maico 250 comparo of an 83 Maico( though I have ridden 250s from Sq Barrels to Radials, Aws, Magnums etc.) About 89 my friend( mate?) took me with to buy a nice 83 Maico 250 Spider. I had an 81RM250 at the time. Soon after, I bought an 89 RM250. We practiced often on a high speed dry slick makeshift track ( about 2.5 mi long) then.
The 89 RM would run and hide from the Maico-as would the 81 RM250 ( to a lesser degree). We also had an 85 DR250 4 stroke. The DR was the ultimate when we raced for ten or more laps around this track-by lap 8 or 9 you would catch the two strokes as they spun the tire and topped out on straights-except the Maico. I am just saying that , I think Maico kept heavy flywheels, conservative porting, and timing in the 250s up to 83.
I did ride a Rage Racing modified 81 reed 250( decked, ported, lightened stuffed crank, V Force CR reed block- Pro form pipe) that was like a modern 250-quick revving and all-very fast- 2 lenghts ahead of my 77440 until top of 3rd gear. Next fastest 250 Maico was a Sq Barrel-1972, and another one -1971. Both of them ported with lightened cranks. The 71 crank lost too much effect, but the 72 bike was primo-an excellent 250 in almost any condition. So, the seatof pants and race to the first turn measure of this-in my experience, is that the flywheel mod and porting can work well if done properly. Yes, I have also ridden both YZ250a, and YZ360( I think that was a B though) too extreme the other way.
This is a very good discussion-especially compared with the US board where we discuss how much a few hate Dick Mann, and what dickheads some of us are for supporting him etc.
Kawboy-keep it going, because if you can get the throttle response up slightly and drag another 2-3 hp out of a radial/Aw 250 -you will have one of the best 250s on the track.
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Thanks for yr encouragement James,
Yes it would be good to have the 76 AW porting specs to see in what direction the factory went in its development of the 250.
Yr comments about the Maico 250s you've ridden are interesting also.
Wrt flywheel lightening specifically on the Maico, DL checked for me & the balance holes are only 3mm from the edge of the flywheel, which severely limits skimming, where the weight loss would have most effect on inertia & is uniformly removed around the flywheel. If you start drilling, there are so many considerations/complications it hardly seems worth it. eg you cant drill near the crankpin cos of pin & balance holes etc, so you'd have to be v careful where you drilled & rebalance etc.
Ideally it would be nice to have lighter flywheels on the Maico, but in practice it hardly seems worth the difficulty
Its probably worth following up w a word about assumptions. The problem is not w assumptions per se. The problem is when they are invalid assumptions. Every engineer knows that assumptions are common in engineering analysis/design/calculations, be it statics, dynamics, fluid mechanics, thermodynamics etc. In engineering terms, assumptions are approximations - educated guestimations. They are invalid if they are outside the error tolerance required or just plain wrong to start with. The better it approximates the real situation, the better the assumption.
A good eg is in expasion chamber design. Tuned length is dependant on the speed of sound which is dependant on temp, which in any engine/chamber, itself dends upon a host of variables. It varies thro the chamber & w time after ex port opens. So what do you do? All the 70's formula just assumed a constant average temp. It got you in the ballpark & you 'trimmed the sails' from there. Even w computer analysis/simulation, you still have to make some assumptions.
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Gents, I can now post the specific time-area calculation results w reasonable confidence. These are not difficult to do (the maths is quite simple) so anybody can check them, but you need to be meticulous w measurements & accounting for direction of flow so you get mean cross-sectional area not port-window area.
Lets be very clear that all I'm doing here is comparing this engine w widely accepted 2stroke theory & practice from the 70's. Nothing more. The values I'm comparing w were determined (empirically it seems) by Yamaha engineers, published in SAE papers, & circulated by people like Gordon Jennings. It is generally regarded as being fairly definitive in analysing such engines
Accepted values/range for the day are (in sec-cm2/cc):
EX 0.00015-0.00016
Tr 0.00008-0.00010
In 0.00014-0.00016
Values we measured/calc'd for the 73 Maico cylinder at 8,000 rpm are:
EX 0.000174
TR 0.000118
IN 0.000103
It quite clearly shows that EX & TR are quite excessive (about 10% & 20% respectively) & IN is quite deficient (to put it mildly - about 25%). Which verifies what I said before.
Or to put it another way,
EX is suitable for peak power at about 9,000rpm
TR for about 10,000rpm
IN for about 5,500rpm
Remember, mean piston speed of 4000ft/min is about 8700rpm, beyond which reliability suffers. Thats why I calc'd at 8,000rpm to allow reasonable margin for reliability.
In other observations, if our measurements are accurate,
EX flange ID cross-sectional area is 10% SMALLER than that of the EX port (15.2cm2 vs 17cm2), &
IN port cross-sectional area (at liner) is 10% SMALLER than that of 36dia carb (9.2cm2 vs 10.2cm2)
Go figure what that does to sonic pressure waves in both, & gas flow!
Interestingly, without the boost-port,
TR time-area is 0.000976 (ie upper end of the range)
Also interestingly, the 74GP cylinder has transfers about 10% narrower than the 73 barrell (tho boost port is marginally wider at the top).
On both barrells, with hi TR, they needed hi EX also to allow sufficient blowdown.
So you be the judge of what is required...
If yr thinking of enlarging the inlet, you can't go wider or you'll open into the TR cutaway on the side of the piston.
If you want to go lower, Jennings hypothesised that you need about 30deg diff between IN & TR timing for ease of kick-starting, so be careful.
For the record, std port opening timings are:
EX 86deg ATDC
TR 113deg ATDC
IN 78deg BTDC
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reading between the lines there are people that want their 250"s to be quicker. yes you could do all the afore mentioned things but i found timing and jetting to be important.........or you could try the barrel and head keith built for me along with the 326 in the early nineties. it uses a 250 magnum piston in a 73 radial barrel. deck heights had to be altered so transfer ports lined up and head modified to get it back to same compression. you can see 4 little cutouts in the fins on the head that he used so he could spin it in the lathe. this setup allowed me to turn the 326 into a 250 but the 326 was a better holeshot bike and i never got to rideit that often although i liked it , better than the less forgiving 400 i tried. performance wise from memory it was quick and had useable power. keith wasnt sure how it would go but i assurred him he got it right. it has been taken out of the cupboard since accidently coming across the 326 article and is looking for a new home.email if interested............geoff
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G' day Geoff, PM sent. Cheers, T250.
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Gents, I checked the area of EX manifold vs EX port again & need to make a correction; the area of the manifold is about the SAME as the area of the Export (both approx 15.2cm2) That makes a lot more sense. (Thankfully I didn't make the same error in the time-area calcs which all still as they're listed above.)
Following up Geoff's post above, it would be useful to know if the top ring on a Magnum piston is any lower on the piston than the earlier 250 pistons (early ones had dykes ring that was almost flush w the top of the piston).
Does anybody know?
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Well Lads after seeing my old 75 250 featured in the current issue of VMX I thought I'd throw my two bobs worth in on this thread, I would never confess to be the most experenced rider kicking around or one of the quickest either but after campaining that old girl for a number of years up here in Nth Qld in pre 78 250 & open classes plus the accational evo events I found it more than adequite, With me in the right frame of mind & the bike prep done properly we would be up the pointy end against guys who I consider better riders than me & on later model bikes with a better reputation ,we did go on to win a couple of events, So as long as the timing was set right & the carb set up properly I cant see why they cop so much flack, The trick I think is to leave the brain on the trailer roll the eyes back and ride it like a 125 (ring it's neck) they are a lovely bike to ride in my opinion & I would love another Cheers Lou
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reply to paul,jc,t250k,on this. measured my 250 barrel with magnum piston.
ex port=47mill diam
in port=37mill diam
top of gudgeon to top of piston =33mill
ring is 2mill from top of piston
i think we are getting confused here. do not have 326 any more... sold whole bike to vic saccottio to add to his pre 75 collection here in s.a. the vintage scene has died a little here since we hava all gotten older but it would be nice to still have a ride sometimes. back to the 250 barrel, head,and piston,does anyone want to buy it. the specs are as above, the piston and bore has done about 1 hour and a few marks on top of piston as i mentioned previously
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Thanks Midway.
Thats the same deck ht as for earlier radial-fin engines, but the top ring is lower which would mean one could retard the TRs (& EX) & advance the IN timing by machining some off the base of the cylinder if one so desired (to correct the time-areas) & used a Magnum piston. Of course the head would have to be adjusted to suit. My guess is thats exactly what Keith did on your barrell/head Midway. Would be interesting to know yr cylinder ht Midway; base-gasket to top of sleeve.
Having spent considerable time over the w/end poring over 2 pre75 barrels (73 & 74), its hard not to conclude that Maico should be ashamed of themselves for this engine as delivered. The quality control & attention to detail is atrocious.
In port heights it seems they were content w 1mm variance from side to side.
In port widths (esp TR) there is an incredible 2mm variance one side to the other!!
On the 74 barrell almost every port is diff in size to the equivalent on the other side.
No wonder the factory riders were said to search for a "good" barrell to start with!
As bad as that is, perhaps the biggest stuff-up is the design of the boost port & its effect on the IN area. Because of the angle, the effective area of the boost port as it enters the cylinder above the piston is 2cmx1cm (ie 2 sq-cm) on the 73 barrell. But the hole in the back of the piston is 2cm dia (ie 3.14sq-cm), necessitating the boost port casting intrude considerably (to put it mildly) into the IN tract & port area.
Had they made the boost hole in the piston 2cm wide x 1cm deep, they would have had a matching 2sq-cm area & more importantly been able to make the boost port casting intrude 1cm LESS into the top of the IN port. That would have given an extra 2.5sq-cm of IN port area (cos of the size of the boost port casting) which it desperately needs.
Also, the IN tract simply splays outward once it comes to the boost port casting (which is 3cm wide) which further reduces cross-sectional area as the charge enters the crankcase. There is no attempt to even keep a constant cross-section of the inlet tract, let alone a 10-15% larger one. Lewo tells me there was some attempt to correct this on the AW barrells, but I don't have the dimensions to know if they corrected it sufficiently.
The unneccessarily large boost port casting simply strangles the engine (esp 73/74) at the point of entry. If nowhere else, it desperately needs work in that area. (And a suitable pipe, as per earlier posts)
As a friend (& owner of one) suggested, it would almost be worth making a new sleeve for this engine to correct its inherent design faults. Then wack on a decent pipe (GMC?) & have a hoot of a time racing perhaps the best-handling VMXer out there - at last w the performance to match maybe.
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Gents,
Lewo sent me the weight of his LIGHTENED Maico crank. For what its worth, I weighed some others for comparison - & listed them below w their dia:
Wt(kg) Dia (mm)
Maico 250 4.2 122
TY250/DT360 3.8 118
DT/MX250 3.0 110
Bul 250 (75) 3.0 106
VR/V75 250 2.6 100
Remember that Maico weight is already lightened "about 10%"
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Very old thread, but why starting a new one when I've just one remark to make .....
If you want the 250 to rev higher and vibrate less, take the lighter crank (3.7Kg) of the models from 1978-81.
It fits with some effort with the case and a new adapter for the ignition. I've used in my 1970 Square:
(http://up.picr.de/17159659wu.jpg)
;D