My OS FS 91 Surpass is several years old, however I have just started using it a couple of months ago. It is mounted in a 4 Star 60 with a 14X6 prop and 10% 2 cycle fuel. I have flown at least 25 flights and it ran perfectly. About 2 weeks ago it started acting up. It would do the following, but not every flight. Sometimes you could tell it would not go to full power in the air (fine on the ground), other times it would just quit runing after being up for 5 or 6 minutes, other times it would cough on take off.
I have changed the glow plug to another F Series, changed out all the fuel lines and it continues to do the same. I am sure it is not runing lean however did notice that at the exact needle valve setting where it had been flying fine, it is only open approx. 3/4 of a turn.
My last effort, have not flow it since I did this was to take the carb off, clean it and air blow it out. I did notice that there is a slit in the copper tubing inside the carb throat - is this where the fuel is dispursed?
I guess what surprises me is it has been running just teriffic and all of a sudden for not apparent reasons is getting really cranky. Can you help?

Check the needle valve. If you see a polished ring around the pointy part, it's likely that the needle seat has been distorted, and your needle's not metering with the needle, but with the end of the threaded part. We usually see needles at about 1-3/4 turns out from closed when leaned to peak RPM.

You may need a new carburetor body, if that's the case.

Please, also check the rocker arm/valve clearances. They will tend to open up a bit as a new engine breaks in.

Bill, thanks for the reply. That makes sense regarding the needle valve. I went ahead and ordered a new carb and am optomistic it will cure the situation. Thanks again.

Rene