The humble 250 Maico

[JC]

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.

[firko]

Would I be so bold as to suggest that you seem to have a bit of spare time on your hands John?
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).

[vmx42]

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

[VMX Andrew]

ummm kawboy can we go through that again..

[090]

Im not reading that!!!

[paul]

mmmmmmmmmm   

[Lewo81]

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 !

[JC]

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.

[T250K]

  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.         

[Lozza]

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.

[firko]

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.

[T250K]

 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.

 

[JC]

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

[firko]

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                                                   
                                               

[JC]

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.