The tan Teflon pump diaphram is reported to pump the best. What carb are you using? Is this magneto or electronic CDI? Have you checked the timing? Have you tried a smaller prop? Are you adjusting ther carb needles only after the engine is fully warmed up? Are you using a heat insulator between the engine and carb?

I get 9300 with a a MAS 15x8 on electronic CDI using Coleman fuel and Pennzoil Outdoor 2 stroke oil at 32:1, through a Walbro 167 carb. The Coleman fuel is reported by some to run hotter than gasoline, but I have not had any sagging problems with the engine fitted to a 1/4 scale Sig J3 Cub. Just my experience. Good luck.