I have an SK90 on a Big Stik 60. The engine runs great except in the following scenario:

I go full throttle into a vertical climb. When in the vertical position, if I throttle back from full throttle in an attempt to hover, the engine will stall.

I know it's not the plumbing in the tank. I checked it. Also, the engine runs fine when wide open throttle during a vertical climb. I also did some trouble shooting on the ground. I went to full throttle and held the plane in a vertical position. Engine runs great. But then if I throttle back from wide open, it dies.

The high speed is already set slightly rich and there's a nice smoke trail from the exhaust. I tried to fatten it up even more but then the engine would burble and not run smoothly at full throttle. I don't think it's the low speed needle either because I can idle for several seconds and then transition to full throttle without any problem.

The interesting thing is this only happens when the plane is in a full vertical position. In any other scenario, I can transition from one throttle position to another without missing a beat.

Any thoughts? One positive aspect is i've become very good at dead stick landings. I can land it on the runway on a dime.