Have you changed to a new plug?

Have you ever reset the valves? .003" on both.

We never recommend anything less than 20% nitro, 20% oil in any YS engine, and the fuel you're running contains neither. The 13x6 on 15% invites problems, especially in hot weather. You don't have enough soup to turn that prop with the lower nitro. You can try a 13x4 and it might help, but the real solution is better fuel.

After you fix the fuel and prop issues, go to www.ysperformance.com and following the instructions from top to bottom. Might help get it right.

Good Luck!