Sounds like mine. I ran mine for a couple of years and it was pretty much a fuel and forget engine. I mean I went several months without touching the needle valve. It sat for about a year and then it started to act all messed up. I read here and there to look at the diaphragm. I pulled mine apart and it was stretched out. You could see where the plunger had been pushing on it so it was not as tight as it should be. I ordered (4) new ones from [link=http://www.centralhobbies.com/Engines/parts/YS63FZEx.html]HERE[/link] item number 71

My settings are regulator screw even with rim, about 2 turns open on the main, YS 20-20 fuel, MS 13X6 prop.

Sounds like you ether have dirt in the needle valve or the regulator is not giving enough fuel and it is leaning out.

Dru.

Edited for spelling