My brother's Ryobi has an 11mm carb, gutted and larger outlet stock muffler, and electronic ignition. No other mods. It turns an APC 18 x 6 prop at 7700.

The stock carb is very small, so there is room for improvement there, and gutting and opening the outlet on the exhaust (muffler) helps a lot.

The first thing to look at if you are going to get inside the engine is the ring. Every used motor I ever took apart needed a new ring. Optimum ring gap is around .003", which is a very tiny gap. It is not uncommon to find a ring gap of .060" or more on a used engine. Frank Bowman rings are the best and almost always add power, even if the stock ring wasn't too badly worn.

The second thing to look at is the exhaust timing. Some Ryobis have a very short exhaust timing, sometimes around 120 degrees. Raise the top of the exhaust port until you get about 150 degrees. (No more.) Don't forget to lightly bevel the edge of the port after raising it, so as to not damage the ring and/or piston.

AV8TOR