I have a YS 91ac mounted inverted that I purchased as part of a used airplane package. The person I purchased it from had it for aprox. 1 year, never flew or started it. It looks like it was flown by the first owner but i have no history. I have a fuel set up with two "T" fittings, using YS 20/20 fuel and the engine seems to be getting proper compression. Here is the problem. When I start it with the glow plug on, it will run, If I take the glow plug off when the engine is less than half throttle it will quit immediately, if the engine is running above half throttle it will continue to run but sputter out when it is bought below half throttle. I am letting it warm up with the plug on and letting it build compression then slowly raising the throttle. The screw that is on the underside of the engine was recessed a bit so I set it flush with still no luck. Also tried many combinations of high/low setting..........Any ideas??!!

Also-what is that screw for on the underside of the engine which is supposed to stay flush and what is that indicative of in an engine when it dies immediately upon removal of the glow plug? Thanks for any insight...