Originally Posted by Rich Browne

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.