With airplanes it is imperative to eliminate as many variables as possible when setting up an engine. Therefore, I always run mine on a test stand first. If it performs well on the stand, but not in the plane, then the problem is definitely with the plane, not the engine. It might be the fuel system, or, in your case, the cowl. But, in order to know that you have to first know if the engine runs properly. It can be very frustrating and extremely time consuming to try and diagnose problems with the engine in the plane.

But, if you don't have a test stand, do this:

Use fuel with at least 18% oil, preferably with a good amount of castor. Triple check the fuel system for pinholes or tears in the tubing (inside and outside the tank). Make sure the centerline of the tank is at or just slightly below the level of the needle valve when the plane is level. Drop down to an 11x6. And, finally, fly the plane without the cowl.

If it still acts up you will need to run it on a test stand to determine if the problem is with the engine.

David