I have twin 5.7L mercs with alpha I gen II drives. Today the starboard engine started to slip bad. Normally when I idle both are around 900rpms, today the port was at 1100rpms. When I brought the boat up to plain the starboard rpms were 500-800 higher than they should be and hardly any power. The engine shifts ok and moves the boat forward (when only engaging the starboard engine) I figured it was a bad prop hub since I had one go bad earlier in the year(hit a stick), but I just pulled the prop and the hub looks good. Any thoughts?

coupler?

I put a new prop hub on today and everythings fixed. The old one had a hairline crack on the top of the insert that I could barley see. Coupler would have been my next check.