Well, I wish I knew for sure. I'm pretty sure it was because of wrong head clearance. When I got the motor and ran 30% Sidewinder in it, I put one head shim in to raise it up. It ran great for 1 gallon and looked new inside.

Rigth before the Nats I switched to a different fuel that was 25% and figured it was OK to remove that shim. Well, maybe it wasn't. My pit guy and I were fairly confident that it wasn't lean (but it was so noisy it was hard to tell). I went out on the track and blew a plug. Brought it back in and put a new plug in. Went out on the track and the motor went to crap. When I pulled it in and torn down the motor to see what was wrong, it was clear. The plug was trashed and the piston had a major gouge in the back side (by the exhause port).

I really need to learn the proper way to check deck height. Because guessing and trashing a $600 motor really sucks.