Originally Posted by GMSchmidt
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.
Check the cir-clip on the exhaust side of the wrist-pin - be sure the pig tail didn't break off and go through the motor. That might be the culprit -not the plug. Check the big bearing - be sure it's smooth - not gritty feeling. Remove the crank and feel it by hand.
Deck height isn't as scary as it sounds - if the plug is deformed add .005" There is no ideal height. You use the deck height as a way of timing the motor. The more aggressive the timing the better the bottom end -down side - you risk detonation. The later the timing the more top end - you do give up bottom end - and you can creat an absolute pig of a motor don't go to far.