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.