I have an ASP 21 mounted on a great planes PT-20. Same engine was used previously with another model without any problems. When I tune the high speed needle for optimum position, I close it until I do not hear any gain in RPM, then I open it a little. My problem is that when I hold the plane 45 deg nose up at full throttle , The RPM drops significantly immediately and engine stops after a few seconds which indictaes a lean setting. When I hold the nose 45 deg down, the RPM increases.

I tried to open the needle and retest again but did not make any noticable difference. My fuel tank is 4oz (120cc) and the center of the tank is around 1/2" below the center of the carb.

I have observed that when I close the tip of the muffler with my finger while running full throttle at 45 deg nose up, the RPM increases and goes back to normal.

Same engine and tank ran perfectly on another plane. Only difference was the tank was positioned 1/4" approx. below the center of the carb.

I replaces all fuel lines and checked tank for any leaks. Every thing seams to be OK. what could be the casue of the problem?

Any advice will be highly appreciated.

Thanks