I have a OS-46LA.The first airplane I put it in was as Great Planes Super Sportster.I call the OS guys and they told me it was nothuing wrong with the engine ,of that pertictular engine it was something built in.Since then I've put it in a Great Planes J-3 And a R/C Nobler andf I got the same result.Its usualy around mid rpm range.Its not the aircraft just the engine.