Just bought an 85 atc250r and about 30 mins into riding the thing i blew the top end. The guy i bought it from said that the top end only had about 4 hours on it, ive never seen a top end that fresh go like that. The cylinder is scored pretty good on the exhaust side. The piston is scratched pretty good on the exhaust side and is missing a small chunk right about the top ring. The piston also has an indentation on the top where it looks like a hole may have been starting. Took the cylinder into the shop and they said that it looked like the last time the top end was rebuilt the cylinder wasn't bored or honed but they dont think that is what caused the problem. It looks like it may have been running lean. After i pulled the engine apart i went ahead and rebuilt the carb, while doing that i noticed that it was completely missing the air/fuel mixture screw, could this have done that much damage? Anyone have any other ideas on why this happened? Its already bored .20 over and they suggest going .40 over with this bore. Is that still gonna be decently dependable? I've heard much over .30 over and it starts to run to hot. I also want to have the head shaved down alittle bit to get the compression alittle higher. How far would you guys go with the shaving? Thanks.

 

Post some pictures. Sounds like it was detonating or it was lean, or both. What did the plug look like?
The only thing that will cause this is detonation. What octane gas are you running?

 

Plug was black, i was running a 32:1 mix. Yamalube oil and 91 octane. I will post pics as soon as possible if it will help you help me. Thanks.