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

From your description of how the engine's running, something's definitely quite wrong. On our test stand, with an APC 9 x 6 prop, 10% 2-stroke fuel, and an O.S. Type F plug, we'll see 10,000-11,000 RPM, depending upon the amount of wear on the engine. With replacement of the piston ring and cylinder liner, the engine should have been broken-in with the same procedure as if it were brand new. It may be that the ring has yet to seat.

Thanks! Got it running great again. The piston ring must have seated right after I changed the cam setting. It was off by one tooth.

Hmmm. Sounds like one of our techs accidentally reassembled the engine a tad off. I'm glad to hear that you found the difficulty and saved having to have the engine come to us for another look.

We're sorry we missed the cam gear!