I have an old FS 48 which I bought new and used only a couple of times a long time ago. It has been in oiled storage since then. Recently I decided to try the engine and it started and ran fine on the ground. I mounted it in a Sig Rascal and flew it yesterday. In the air the engine speeds up, slows down, speeds up then eventually quits. Any ideas? I checked the tank and plumbing and all was ok. Engine is inverted and is running on 10% nitro with a 12x6 prop. I don't have a tach so could not measure RPM. Thanks for any help.