Okay, here is the deal. Today, in my third moto, my bike (2000 YZ125) started acting a little funny. It didnt want to pick up RPM, and wanted to stall. It seemed almost as if it wanted to foul a plug. I ride it out anyway, thinking it is just a fouled plug. I pull off the track, and my bike just dies. My dad and I push it up, change the plug, and go to kick it, but the bike has no compression. Late tonight I pulled the top end, and part of the exhaust side of the piston is missing. It looks as if it was burnt off. The entire top edge is very rough. The exhaust pipe o-rings went bad about a week ago, and I havent had a chance to replace them yet. Our theory is that the backpressure into the cylinder sucked air through the exhaust/cylinder junction. There is very strange buildup on the piston. The top and exhaust edge are dark black while the rest of the sides are brown above the ring. Today was overcast and a little cold at the races, so the bike may also have been running a little lean. Whats up here? Could the pipe o-ring cause this piston melting? The exhaust side melted about to the inner edge of the piston ring. Thanks in advance for your help!