Those RPM's don't sound right for your combo, if your running a tuned pipe, my guess is its too short, and not coming on the pipe. You might also check your head clearance. These are some generalizations based on my experience; a stock engine with a regular muffler and 15% fuel, set the head clearance at .010 to .014. As RPM increases to 18 to 20 k, increase head clearance to .017 to .020 to prevent detonation.

You can tell if your engine is detonating using a tach. You will get very eratic readings, jumps of a few hundred rpm will show up, you will also see pitting on the piston top or inside of the combustion chamber, (bottom of the head).

Hope this helps.