Idmep is on the right track. My .91FX, and my friends' are both set up with Pitts mufflers, and had problems with deadsticks. In both cases, we found that running with 10% cured the problem, while the engine would die in ther air, after a few minutes, on 15%.

We're both running ancient Cermark Sukhois, cooling airflow is not an issue, and we're both experienced with low- and high-end needle adjustment. We tried props from 13-8 to 15-6; I use the 14-6, my friend uses the 15-4. The big difference came when switching to 10%. On 10%, they are reliable.

I'll also suggest you make your needle adjustments with a half tank of fuel.

Good luck,
Dave Olson