Are you using a tach to set the high end ?
If so what RPM are you getting after a about two minutes at wide open throttle ?
On this engine (even though it is not fully broken in) I would expect close to 10k sustained RPM with a MA 13x6 prop.
It should peak greater than 10k with the MA 13x6.
The Magnum engines I have do not really run well when they are set to rich, and sound like they are missing once in a while.
Unfortunately they will also sound like this if they are to lean.
If you don't have a tach I really recommend buying or borrowing one.
As was mentioned earlier I too have found that new engines are prone to eating a glow plug.