ORIGINAL: bigchap
inverted engines always run better with an os'f' plug fitted.
Yes, that's what I was getting at when I said a "hotter plug". The "F" is the hottest and is what I run in my four AND inverted two strokes. Everything has to be adjusted perfectly and the engine warmed up. It takes some tinkering, it will pretty much stay once you have it. Except the few clicks either direction depending on temperature and humidity (overall air density).
RC Outlaw brings up a good point. Rotate the prop with your hand to make sure the engine is not hydrolocked. If you let your electric starter kiss your spinner when hydrolocked you will likely damage your engine.