J/30 Class Association Forums J/30 Forum The Boat, Sails and Equipment Motor mount replacement

I'm getting a lot of vibration at idle in neutral and at all speeds in gear.
I had the prop rebuilt and replaced the cutlass bearing three years ago, which helped with the vibration I was feeling then, but it has gotten worse.
I plan to re-check the shaft alignment, but I don't think that's the major problem since the vibration is just as bad in neutral.
I have observed a lot of movement in the motor mounts. They aren't cracked, but could just be fatigued after so many years (1985 model).
The only thread on this subject is from 2007, so there's a lot more wear from the ensuing years.
Has anyone tried replacing the mounts? The front ones are pretty accessible, but I can't figure out how you can get to the underside nuts on the rear bolts of the rear mounts.
Would appreciate any advice.

You may have a cylinder that isn't firing. Check the injectors for good spray (pop test). Also check the fuel pump output and timing. If you don't have experience with a diesel engine, a marine diesel mechanic can do these tasks.

Having the vibration in neutral would rule out the shaft or prop. I would suspect a bad spray or clogged injector. Dan #409

Thanks for your advice. I found that alternately pushing the decompression levers cause the same reaction with either cylinder, so it's running on two. I haven't yet found a mechanic to do a pop test, but I'll pursue that on Monday.