I had this same problem on an O.S. 91 FX mounted inverted. The engine would run fine with the glow starter attached, but would quit after about 5 seconds with it off. I tried everything with the needle settings, etc. It would seem to run fine at full throttle for a while, but at anything less than full throttle, it would slow down, and then quit. Like throwing a switch. I took this engine off of the plane, bought a Saito 100 and put it on. It runs perfectly. ( I really wanted that one on the plane anyway, good excuse to buy one!) After this, I bought an OS .25 FX brand new. I did not have a glow plug for it, so I pulled the one off of the 91 and installed in in the OS .25 FX. I Started this engine fine, and was running it very rich, so I expected the engine to quit when pulling off the glow starter. Once the break in was done, I was trying to get a good idle, but it would quit on me the same as the 91! I suddenly dawned on me that the same glow plug was in both engines! I replaced this plug with a used one from another engine, and viola! Purrfect idle, great transition, and will run all day at 1900 RPM. This was a brand new, in package OS #8 glow plug that was bad!