Something must be wrong with my 91FX! CP 15% only gets me 8800 on a Pro-Zinger 15-6. I have a Pro-Zinger 15-4 to try but I needed another washer between the prop and stock washer. The nut ran out of threads before it would tighten up on the prop. I bought an Ultrathrust muffler for it but haven't used it yet. I need to get mine up to the performance you guys are getting before I swap mufflers. Maybe it's due for a piston ring. Compression is excellent and it starts and runs fine, just not near the power you guys are getting. I'm pretty sure it used to turn over 10000 with the Pro-Zinger 15-6 last season.
Matt