The YS 4 strokes run excellent inverted. Most others tend to have problems. Alot of people run the 4 strokes not quite inverted but at a 45 degree angle from bottom center. This keeps fuel from pooling on the plug. You tend to have to run a leaner low end to keep them running. The easiest way around this is to add an onboard glow system. When the engine is below 1/4 throttle, the onboard glow driver kicks in and keeps the plug lit. Good Luck!