OS has great service - if you follow the manual and something bad happens, they will replace it. The instructions for breaking in say to run rich for a few seconds, then lean out for a few seconds, which allows for heat build up, expansion, and full range motion.

My manual for the smaller AX engines say to have the low needle one full turn open from flush with the housing, but apparently that has been changed. This is the excerpt from the online manual:

Close the throttle rotor gradually from the fully opened position until it is just fully closed. (Do not turn further.) Then, screw in the Mixture Control Screw until it stops. Now unscrew the Mixture Control Screw approx. 3/4 turn. This is the basic position.

This is the same as my 120AX, and indeed 3/4 turn open is correct (for 15% nitro fuel).

When in doubt you can also call Hobby Services. They are very knowledgeable and kind, and most of them even fly planes.