Leak exhaust port into crankcase

[dirtbike]

I did something stupid of course. Good thing, I learned something again. Bad thing, It was probably not my last mistake.

This a little out of topic since it's about my maico but it's a general two stroke issue really. That's why I take it here.

The Maico 400 and Suzuki RM370 pistons are identical. Wössner as well as Wiseco sell the same piston for these 2 bikes. I use a stock, and stock bore Suzuki RM370 piston in my maico. When I mounted it, the skirt touched the flywheel in BDC. I shaved a
few millimeters away with my dremel. The bike has not been running too well since that. Hard to start, suspected bad jetting, bluddering and four stroking, but pulled nicely in higher revs of course. This has almost driven me crazy. Everything new and really perfect, yet not running good.

Today I saw something I missed before. When the piston is at TDC, the skirt opens the exhaust port into the crank case by maybe 2 - 3 millimeters.

I can almost figure out myself that this is the problem to why it hasn't been running as nice as it should, and did before. Does anyone have experience of this? Or can it be that this really doesn't matter, and that I have other problems?
Opinions?

Thanks
//Bengt

[t20sl]

Bengt:
I believe you answered your own question.  If exhaust port is open to crankcase you are creating an air leak which would lean the crankcase mixture I would think. Would not run as good as it did but I would not think blubery but lean. My question is if Wiesco sells same piston for both why did your RM piston hit the crank in the first place?  Maybe Wiesco has a slightly shorter skirt but not as short as yours.
Ted

[Richard]

Could you have mounted the piston backwards? The skirt on the intake side of the piston is generally shorter then the exhaust skirt.

[dirtbike]

Wiseco and Wössner pistons fits both bikes, but I am using a stock Suzuki (only) piston. They probably didn't care if it did fit the maico or not. I shaved a little off the skirt and took away too much.

Of course I didn't mount it backwards.

Anyway, a new one on ebay can be found for $20 - $40.

[Richard]

My apologies.

[john durrill]

Bengt,
Do you have a stock Maico piston to compare the RM370 to?
You could slip a wrist pin half way in the stock piston , slide an RM 370 on the other end and see how things line up. The wrist pin hole could be cut in a bit lower on the Maico's piston.
It must work because you had it running better before the skirt change.
john d.

[dirtbike]

Richard, no need to apologize. That could very well have been the case.

I have lined them up using the pin just as you suggest John. But since I cleverly enough did cut off the bottom of the skirt I cannot see how much too long it was before. All I can say is that the stock Suzuki piston i a bit longer on the skirt, and when I did this I didn't pay attention.

But really, the question is if anyone have experience on how bad the engine can run when the piston opens the exhaust on the underside a bit?

[tomale]

Bengt, I am really not convinced that the gap into the exaust port is the problem, specifically, Piston port motors loose potenial power because of this but the same can be true of the intake. besides if it was a problem I would think that it would cause the bike to lean out not blubber. With the new piston you are creating more vacuum into the motor and this will change your jetting, cooler weather will cause this too. If you are running a Bing Carb, and now with a new piston, and if the needle jet and jet needle have not been change, it will make your motor run just like that, I know, it happened to my Maico several years ago. It ran great on the top end but wanted to load up on the bottom end. It was hard starting and was frustrating to ride.. If it has been more than a year since you replace those two jets, it is time. unlike a Mikuni the bing jets do wear.. all I did to make it run good again was rebuild my carb and wham, the bike ran good again.. hope this helps.

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)
74'250 hare scrambler (project)
74' 1/2 440 maico
78' 440 maico
72' cr125 Husky (project)
93' RMx 250 suzuki

[Ed Chesnut]

There is no way the engine will run correctly with the short piston skirt opening the exhaust port from 3mm BTDC till 3mm ATDC.  After you get that problem solved, then you can address other possible issues.  This situation is a PRIMARY problem which must be properly resolved -- i.e. new piston which does not open the exhaust port to the crankcase volume at Top Dead Center.

Ed

Keep the rubber side down!

[dirtbike]

I can understand that It's hard to really guess what is happening, since I did cut off a little too much all way around. Now, as I said, the exhaust opens into the crank area, but the port timing on the intake is of course way off too. I have basically ported it for high revs cutting the piston, combined with this exhaust opening it really doesn't perform very well [B)]

Anyway, problem identified and a new piston should do it. I have replaced all jets and the needle in the carb as well.

I will adjust the skirt using the John Durrill "line'em up" method this time [:I]