re-occurring cylinder prob

Heat.

Methinks that is the problem. Not just heat, but excessive heat.

Too lean of a mixture could be the culprit. The leaner the mixture, the hotter the burn. Over a certain limit, and it's bye bye piston.

Not enough back pressure from the exhaust can cause a lean mixture, since most of the fuel will just blow straight through the motor.

Too hot or cold of a spark plug could possibly be the culprit as well.

Carbon build-up can cause hot-spots, so better fuel is best I guess.

I had a muffler come apart on me once. Due to some incorrectly performed repairs, I had like zero back pressure. (This was before I realized how important back pressure is to a two stroke.) Within a week of riding it like that . . . PANG! CHING!! CLING! CLANG!! put put put put put sputter DEAD. Took it apart to find that a piston ring had completely disintegrated. There was the same type of black goo that you mentioned, along with scars all inside the cylinder walls.

Anyways, 'tis a mystery. Good thing is, you should be able to use a process of deduction based on what is different from the norm on each bike. If there is a deviation from normal engines that occurs on BOTH of your engines, I'd say that that particular deviation is likely the culprit.

Good luck in your engine sleuthing adventures.

Hope you not running the high RPMs or flat out while going down hill or while your breaking in a new motor. Another possibility, how long has the two stroke mixture been sitting in the can or in the tank? Two engines back-to-back is suspect that maybe you doing something wrong.

Good Luck,

Chris
AKA: BigBlue

So do you use premium or what?

I use ethenol gas, tried many premix's. I don't have same engine but is a 2 stroke.

I have raised the compression slightly as most suggest and the firing pattern by removing spark plug crush washer like people do sometimes. (We do that on mazda rotaries keeps the plugs cleaner)

Anyways on many performance cars I've noticed too high octane causes higher heat, due to a lean condition if their not tuned for it or an ECU that advances till it senses knock.

I tried high high octane with a booster that'll do 3-4points ontop of summer 91 which can range, and the engine ran piss poor compared to 87. The odd thing is it runs fine, but just doesn't sound happy. Sort of like engines that don't like 4 stroking, this one doesn't mind it like chainsaw motors, but it feels like 4 stroking almost but your not. The higher octane I never saw any perf difference till 4k or more RPM roughly.

The motors exhaust seemed to be hotter and the engine would start running slow. This engine with 87 and 70f outside the exhaust feels just slightly warm at idle, it was hot feeling whenever I ran higher octane the 3-4 times I have.

I just have one question, do you put the clutch in when your not accelerating or do you let the engine deaccel with the chain drive?

BoDean_LP said:

I had a muffler come apart on me once. Due to some incorrectly performed repairs, I had like zero back pressure. (This was before I realized how important back pressure is to a two stroke.) Within a week of riding it like that . . . PANG! CHING!! CLING! CLANG!! put put put put put sputter DEAD. Took it apart to find that a piston ring had completely disintegrated. There was the same type of black goo that you mentioned, along with scars all inside the cylinder walls.

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Interestingly my roommate ran his without a muffler for months without an issue (his was mounted on a chopper and the exhaust hung too low, he ended up cutting it off around the 4" mark from the cylinder). Even checked the cylinder and it was perfect (thats why I'm using it on mine now, as well as being stock tuned). Piston was a bit black but otherwise great condition, LOUD as all **** though (it's deafening when I went WOT).

However I think you're correct on the heat/lean issue. Apparently you're not supposed to use synthetic on break-in because it's too slick and doesn't allow the piston to seat properly. I always try to run mine rich (4-stroking rich) on break in to ensure the motor is well lubricated. And I've never gone easy on my motors during break-in, aside from freewheeling down hills, rather than powering down them. I havent put as many miles on as some of the veterans here, but I'd guess around 200 miles, constantly WOT, only problem is I hear ringing coming from inside the motor, I suspect the wrist pin bearing's about to go so I'm trying to get a good replacement ASAP.

The bearings came apart in your motors? Pretty common with these little Chinese two strokes. http://www.google.com/cse?cx=partne....q=bearings came aprart in my H.T.&gsc.page=1

LeTweek said:

Yes. I've been running premium. I allow the chain drive to help decellerate minimally. I coast down most hills. I'm hopeful that part of my problem was trying to breaking-in new cylinder/rings with synth oil.

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I can't tell you how bad deacceling with these 2 strokes is, but I went with a centrif clutch one so I'd never have to worry.

I read here a lot you should push in the clutch if your moving, and not using the engine and shut it off or let it idle.

My 49cc two stroke Morini engine has a centrifugal clutch and it naturally engine brakes some when I let off my throttle. Darn thing has run flawless forever now..