Ive finally built a screamer (almost). When I first top out my 66cc motor from the start, I'm hitting about 8500 rpm(36mph). I have a high compression head, I've raised the exhaust and redirected the transfer ports and installed an expansion chamber. I'm running a B7HS plug. Here's the problem: once the motor gets good and hot, I lose about 1000 rpm (32mph). Is this because it's running too hot and I need to go to a cooler plug?.....or a hotter plug? NGK makes a Range from B5HS-B10HS. What else could it possibly be? Anyone have any insight?