The humble 250 Maico

[090]

I don't trust that seller. I reckon he is a bit dodgy ! It's probably a Honda painted green.

[JC]

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.

[paul]

then how did the next model 250 engines  go like the aw75 76 77

[firko]

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.

[JC]

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

[firko]

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.

[JC]

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??)

[Lewo81]

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.

[JC]

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.

[Maicojames]

I'll pull a barrel this week.

[JC]

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.

[Lewo81]

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.

[firko]

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 . 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.

[Maicojames]

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.

[JC]

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.