How old is your engine?
What make of ignition is on your engine?
Do you use proper fuel? (With 1:40 oil mix) and what oil quality?
Muffler system used?
Is it in flight or on a test stand?
How are your needles tuned?
Is the front seal lubricated?
Is the vacuum pulse connection to the carb still OK?
Cooling issues?
Is the battery voltage that reaches the ignition stable?
Plane failsafe function?