Re: 4 stroking
I had the exact same problem with a brand new motor. No other signs of air leak besides overheating. Plug looked fine, ran fine for ten minutes at a time, then refused to shut off and began dieseling.
Turned out to be a bad crank seal that had been installed improperly.
Look for the air leak.
If more is better, then too much is just enough.
If you can't afford it, build it yourself.