Losi_racer I would suggest checking the carb for an air leak, If you can wait a few weeks, I know I will be out racing or hanging out atthe nitro pit and I can look it over and help you get a tune on it. Like stated earlier I believe it to be a idle gap issue causing the tune to be off.

Jnc, It sounds like you have had the tune close, how much fuel is thru the engine and are you running the extra head shim. If you look for smoke on the bottom and listen to the sound of the engine on bottom and top. (it may be easier to do this on the bench). Its possible that you are lean on top or overly rich on bottom. I would see if you can borrow a temp gun just to check the temps it make give you an idea of which way to go. Also what plugs are you using, is this a common problem or did it happen once.