My piston failed (lean main jet) and a piece broke off (see pic). When I stopped I noticed an oil/coolant mixture was dripping for a few seconds and then stopped. I suspect it came from the overflow on top of the tranny. There is some also antifreeze and oil where the crankshaft is. I know this was the result from the piston failure as I often check the oil and it never had coolant in it.

1. Can anyone explain how the coolant got into the crank case and the oil??

2. It is my first time doing a top end, and i noticed it is already at 2mm over so I have to resleeve the cylinder, right? I'll go back to stock so I have room for cheaper rebuilds in the future.

3. I will also be splitting the case to clean out the inside of any debris that fell down. The rod has no up and down play and feels solid so i will be reusing the same crank, but i'm replacing main bearings and seals. When i put the cases back together, what is best to use: gaskets, permatex ultra blk, or permatex ultra grey?



Any other tips?

 

What machine?
That piece melted off. It's called detonation.
Providing it's a stock motor (stock compression) It's prolly a poor fuel problem.
More answers will depend on the machine.

 

Sorry, I don't know how I left this out...

It is for a 91 LT250R. I looked up the piston number that was stamped on it and it was a 69mm, 2mm over the standard. That is the max size sold by wiseco, so I guess I should resleeve the cylinder.

And yes, I believe it was a fuel problem as well, this happened at WOT going 60+. When riding trails at slower speeds and throttle it smokes and spews oil so I know the pilot is sufficient. BTW it has a 39mm PWK.

 

MYSTERIOUS TWO STROKE SEIZURE



Dan's Motorcycle


Your running along on your dirt Ring Ding and your not pushing it at all. Nice day, 1/2 throttle, grooven on the scenery, eat'n up the miles, head'n home, and then it happens. The bike locks up and you skid to a stop. Why ? ? ?
I've seen this a lot of times. The story varies a little but basically you were not pushing it, but you did have the throttle in one position for a long time. I've never heard anyone address this issue other than to say "Don't run your two stroke, for long distances on the street." I believe it really boils down to engine design. An engine is designed according to it's use. A two stroke lawnmower or outboard engine is designed to run at a steady speed. A two stroke motocross engine is designed to be run full throttle, part throttle, full throttle part throttle. On and off, on and off.
Most two strokes use what is called loop scavenging. This refers to how the incoming charge flows into the cylinder and how the exhaust flows out. When an engine is run at a steady speed, I think the temperatures tend to stabilize and stay the same at different spots in the combustion chamber. It stays cooler at the intake ports and hotter at the exhaust ports. This is no problem as long as the temperatures stay lower than the melting temperature of the piston. That is the problem. If the engineers design the engine run at a steady speed, they make sure the temperatures are low enough to not cause problems. Motocross engines are designed for on and off running. I think that getting on and off the throttle all the time tends to mix the cool incoming mixture with the hot exhaust gases. This cools the area over the exhaust port and keeps the piston from melting. Run the engine at a steady speed and the area over the exhaust port starts to get hotter and hotter until Boom, things melt. The problem is the steady running. If you run steady at half throttle it will take you longer to burn down than if you where going at three quarters throttle, but sooner of later it will happen. At least that's my theory as to why it happens. Bottom line ? Don't run your dirt bike at a steady speed. If you have to run on the street keep working that throttle on and off.

 

...or make sure your jetting is rich enough.