Sounds like you have a hydraulic lock, which is difficult to avoid with an inverted engine.

All I can say is try to avoid the fuel getting to the carb too soon before turning it over to start. Don't forget it can run back into the exhaust port from the muffler after fueling.

Actually, I always start my 91 FX using a chicken stick having slightly over fueled it, and flipping it the wrong way. It starts almost every time, but, admittedly, it's not inverted.

-David C..