OK. Still need to make sure that you have 3 times exhaust area compared with the intake in the back of the cowl. This will avoid the hot air to remain inside the cowl and overheat the engine. Check or verify also the following.

1. Make sure that is not running lean when uses some fuel. How you adjust the engine?
2. Check tank position. I assume that you don't have a pump.
2. Is the engine new? If not it could be bad bearings.

Good luck,

VB