On first running my new 912 uls a couple of weeks ago, I got a mag drop
pf just over 500 rpm on one mag and just under 600 rpm on the other mag,
starting at 4000 rpm. All else was OK; oil temp and press, EGTs, CHTs
etc, and the engine ran quite smoothely; even on 1 mag with the rpm
drop it was not obviously rough or misfiring. Prop is Airmaster,
operating correctly - testing done on fine pitch.
We chased a similar situation with our 914, Keith. Documented in the
archives here:
http://tinyurl.com/76oxud
Long story short, our issue was with the carb needle settings (... set
too lean as delivered from the factory).
Good luck!
D