This is a ringed engine and should be run quite rich for a gallon of fuel or so, and after that a lean run might ruin the ring. If it only turns a 12x8 10000 rpm return it to get repaired, it should turn it around 11500. Also try a larger diameter lower pitch prop if you would like more vertical performance, like a 13x5,13x6, or a 14x4. The high rpm number they list is usually the maximum rpm (redline) not the recommended rpm.