All my four strokes run fine inverted. It's all in the low end tuning. I finally got my brand new OS 120 to idle for an extended period of time after about half an hour of tweaking the low speed needle valve. The trick is to adjust the low speed needle valve while the engine is idling. Set the high speed needle valve first, then slowly lean out the low end and make sure the rpm increases while you are doing it. If the rpm starts to drop or the engine suddenly quits while you are leaning it out, then you are too lean. Richen the low end by 1/8 of a turn, restart the engine, and check the idle again. If the rpm drops significantly when you remove the glow igniter, then you are too rich. Eventually you will find a sweet spot. Just keep on trying. An on-board glow driver is nothing more than a bandage for sloppy low end tuning.