Please help w ideas, I was having plug problems but pretty sure power valve solved that... Motor shakes boat up to steering wheel then smooths out at last 1000 .rpms... Replaced motor mounts and took off bravo to use alignment tool, boat vibrates in neutral when reved up

is ehe engine dowd on power,like it is missing on some cylinders?

Your harmonic balancer may be shot...

You can check the balancer by; removing the crank pulley, bolt and washer then check to see if the crankshaft key aligns with the TDC or 0 degree mark on the balancer. If the engine has been rebuilt and internally balanced, someone may have installed the tungsten, improperly and it has come out. That's a long shot, but nothing surprises me anymore.

dont know what kind of drives your running, but i had a pressure plate come apart in mine!

did this motor start vibrating after some time running smooth? or vibrated from install? cuz it sounds like a balance issue. i put a 400 small block in an old dancer to try to get it over 40. guy sold me 454 flywheel-said it was 400 sbc. shook like my old triumph at idle. you could see bow rails shaking about 4 inches back and forth. if it isn't the balancer, the motor probably has to come out...

with fouled plugs and now a shake could it be possible firing order is incorrect.maybe a couple of mixed up plug wires.

The firing order was checked, i got all plugs firing bc boat reved to 5000 rpms running... All the motor ?s the builder should know, I used an alinment tool on install and put new motor mounts on second time pullling motor.. A guy was sayng could be the ujoints, but with last motor the boat ran perfect..

I had the motor out of the boat on a stand and held a flat screw diver against coupler output shaft and reved up motor the screw driver did not vibrate or bounce... With that said can it possibly the internal part of coupler out of whack, so when it has a load on it, it then vibrates...

Thanxs for help ended up. Being flywheel and balanced needed to be neautral