I HAVE THE SAME PROBLEM WITH MY OLD FS-91 4-STROKE I BOUGHT USED, I JUST CAN NOT GET IT TO IDLE BETWEEN 2000 & 2500 R.P.M. AND I JUST CAN NOT GET IT TO DO A SMOOTH TRANSITION FROM IDLE TO HIGH R.P.M.

I BURNED A EXHAUST VALVE, WHICH I THINK WAS CAUSED BY A BROKEN VALVE SPRING.

I WENT AND REPLACED BOTH INTAKE & EXHAUST VALVE ASSEMBLIES, CYLINDER LINER, PISTON RING AND GASKET SET.
STILL CAN NOT GET THE ENGINE TO IDLE. IT SEEMS TO RUN BETTER FROM IDLE TO FULL POWER AND IDLE BETTER IF I LEAVE THE GLOW BATTERY ON. WHEN IT IS REMOVED I LOOSE THE SMOOTH TRANSITION FROM LOW TO HIGH POWER AND THE IDLE GETS RICH AND QUITS.

MY FUEL TANK IS ABOUT 1/4 IN. ABOVE THE CENTERLINE OF MY CARB CAN NOT GET IT TO LINE UP BECAUSE OF THE ENGINE CENTERLINE AND THE PLANE DESIGN.

I AM ABOUT TO GIVE UP AND JUST BUY A NEW ENGINE BUT AFTER READING YOUR REPLY, DO YOU THINK I NEED TO BUY A NEW CARB.?

I USED WILDCAT FUEL 10% NITRO 18% OIL, 15% NITRO 18% OIL AND YS 20% NITRO 20% OIL
PROP SIZE IS MASTER 14X6 AND PCP 15X4