The key to reliable inverted running is a correct fuel mixture at idle. My Saito needed to have the low speed needle turned an entire turn before the idle was correct. Unlike two strokes, a four stroke engine's low speed fuel/air mixture can be stupid rich and the engine still will have almost instant throttle response. The only clue is that it wants to quit at idle and has a voracious thirst for fuel.

I have also seen people who couldn't keep their inverted two strokes running for love or money. Unfortunately, I am usually helpless to help them because they have flown model airplanes for (fill in the blank) years so they obviously know what they are talking about.