Regards break-in...
I had a project for a friend, to do initial break-in on a sizable number of one model of OS engines. What I observed has also appeared on several of my own and a few friends' OS engines since.
First start is often, mmm, shall we say "reluctant?" Exhaust oil is fairly dirty for the first two or three short, heat-up runs. Fuel usage also is pretty thirsty. After 4 or 5 brief, bench runs - to fully heated throughout, then fuel-line pinched off - exhaust oil runs cleaner, starting is easier hot or cold, and fuel 'economy' starts improving.
At about that point, I'd consider them safe to fly, provided you don't go over lean. Too rich is also a serious no-no...
Economy and power continue to improve, and starting stays easy. ...for a long time...
I figure from this that these engines need some carefully controlled initial runs to smooth off tooling marks and any overly tight fits. These are not very large, but large enough to show the symptoms, above. Polishing-in the fits at operating temperature completes the desired break-in.
I've also noticed that the smaller OS engines, often run in low-2 cycle, benefit from leaving the plug connected until they audibly 'lock-in.' A few seconds is all it needs to bring them to full heat. A premature launch sometimes seems to prevent them from 'getting on the step.'