Hi I have an OS 26 FS that I put in a plane and was unable to get consistent top end. It would act lean/rich w/o changing anything. So I would run it rich and it would have low power, if I ran it lean it would quit. There was also a knocking or detonation type sound. It would idle well and would act rich through transition from idle to WOT, even with the idle screw completely out. Checked valve clearance, exhaust valve had too much clearance, still same issue. So I sent it back to Hobby Services they replaced bearings, sleeve, and piston ring. Now ran about 6 tanks through it. It idles well, no more rich transistion, and no more pulsing(rich/lean) or knocking. But... it only turns 8k rpm with a 9x6 prop. Checked the throttle, it is open all the way. Removed carberator, looks clean. Checked the valves, changed glow plug and tried a friends fuel. Have tried pressurized and non pressurized. None of this has made a difference. Still 8k rpm (very consistent at wide open). HELP! Thanks