I think Jim is correct. If the engine is not running sloppy rich, it will probably overheat in the early break in stage. Remember, there's no airflow except from the prop, which may not be enough. A very unscientific way to tell if it's over heating is to wet your finger and tap the head after it stops running. If the moisture from your finger sizzles, it's too hot. You should be able to tap your dry finger on the head without burning your finger. Hot, but not too hot to touch.

Paul