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

It sounds like two issues to me, one is simply tuning and the other is the distance form the tank to the engine (not the height difference when the plane is horizontal).


i) If the engine speeds up when you lower the nose (from horizontal) it means that the engine is set/tuned too lean.


ii) If the tank is mounted far from the engine, then the height difference when the plane is tilted up will be too large. You need too mount the tank closer to the engine and/or increase the pressure from the muffler (using an OS muffler with baffle for instance).

Thanks I will try your suggestions and post the results

I tried inserting a nut in the bolt hold the 2 piecces of the nuffler and fixing an insert in the muffler tip to reduce the internal diameter. It seems that this has helped in increasing the back pressure from the muffler and the pressure of fuel coming from the tank to the carb. it takes the engine now 10-15 seconds to lean out when holed at 45 deg nose up position which is enough for me.

thank you all for your help.

Sounds like the tank is too far away from the engine, it is usually better to fixed the problem rather than just reducing the symptoms....

Hi!
Agree!
The engine should not go lean at all when holding the nose of the plane straight up! It should be able to run at max rpm at that position all day long