Does it smell hot?
If you have good airflow, then it's either the mixture (gas to air ratio) or the fuel/oil mixture. Most of the guys I fly with break in on an ashless oil at 32:1 for a couple of gallons, then switch to a good synthetic like Redline or Royal Purple at 40:1. I know some oils say you can run as high as 100:1....but given the choice I'll save my motors and run at 40:1.
For airflow you must have an exit for air on your cowling....they say the exit is supposed to be 3 times the entrance area...I break that rule alot.....I run maybe 2 depending on the model.....
Airflow is like water it will take the path of least resistance so you need an open area in front of the cylinder and an exit aft of the cylinder.....If the exit is too small then airflow will dam up forming positive pressure letting no more air in and then will heat up as well.
Good airflow may require you to damn up an entrance into the cowl to force air over the cylinder...
Good Luck