RE: synchronizing engines
Have you tried moving the props from one engine to another. swap the prop from the engne with the lowest rpms to the engine with
the highest rpms. I have done this on a twin and it got the rpms closer. I tune each engine then check rpms,then try to get the rpms closer by swapping props around to get them as close as I can,then I take the prop from the engine with lowest rpms and sand the end of the prop down a little at a time.this lightens the prop up and reduces drag at the tips letting it spin faster.to increase 200-300 rpms does not require much removal of the prop tip,then recheck balance.