New Motor Mounts or ?

[mjmonk49]

My Yanmar 2GM 13Hp has a rough spot from 1200 to about 15oo rpm when it is under load. The motor jumps around on the mounts quite a bit, but smooths out at about 1600. Starts right up, uses no oil between changes, new filters and fuel has been de-bugged. I even put a dial indicator on the shaft and it runs out less than .005 when turned by hand.
When not under load, runs right up to the 3200, no hesitation, no rough spots and sounds great at all rpm's.
A mechanic friend looked at it and said it was one of the best yanmar 2GM's he had heard. And after physical inspections deduced that it might me the mounts. My question is, could it be anything besides motor mounts that could contribute to this shaking when under load? Thanks

[a64pilot]

Most engine / drive combinations will have certain RPMs that the harmonics line up to cause vibration.
Usually most Boat manuals will recognize this and tell you to avoid those RPM ranges.
Usually you can inspect the mounts for sagging, and splits etc and tell if they need replacing, hardening from age you can’t really inspect.
Mounts are different, their hardness is “tuned” for specific engines and it’s common to have different mounts on the same engine based on position.
If you decide to change them I think I would use Yanmar mounts specific to your engine. But sit down before you price them