Bax,

The engine is a brand new, Chinese product, that I just purchased from Tower. I do not see any sort of insert inside the carburetor. I am using a Bisson Pitts-style muffler designed for that engine. I am using standard (medium?) size silicone tubing. I started out with the factory setting on the low-end and very rich on the high end for break-in and I have since leaned out the low-end about 1/2-3/4 turn and the high end I've leaned so it runs smoothly but quite far from its peak RPM, and it still leans out in the air every time, especially during high-G maneuvers.