I purchased a new GM 502 marine base engine from 800 runs new for my 29 Fountain. I installed it and get a bad vibration over 1500 RPM. So far: Changed Flywheel from Summit to one provided by 800 runs new. It's the right flywheel. Changed coupler. checked plugs, changed wires. the distributor is a band new HEI from DUI and appropriate for the engine. The carb is a Carter 4FB that I had built to the spec for the Merc 502 with the correct metering rods and jets. I have run the engine without the drive, disconnected the water pump, power steering pump, and alternator independently. I checked the timing and spark plug wiring three times. I have checked the mounts. Does anyone have any ideas? I am losing my mind!

Thanks,
Chrisholm

had the same issue on a 454 that I bought brand new, very noticable vibration, could not find anything, was in a repower on my 50 Gibson, which never saw 4000 rpms, lasted 2 years then spun 2 bearings for no reason, took apart and found had a scat crank, yes in a new GM engine, posted on here about it and found out was normal(GM does not assemble the marine engines, off site vendor) so bought another (factory balance) crank from scat, this time had engine balanced and shop barley was able to get right(chinese junk) but did and has been fine for the last 2 years...so that being said, bet the engine balance is a joke...sorry for the bad news...one other thing, did you make certain flywheel was installed in correct position ?

Pull engine and take it to an engine shop. Or call Runs new and tell them to replace the current one

I agree, check flywheel. If its missing the dowel pin to line it up it will vibrate bad. I did that several years ago....

The flywheel dowel pin came on the engine, so I know that's not it. 800 Runs new is telling me to do a compression test. Then if that's OK swap carb. Thanks for all the responses, although I have to say I don't like the answers all that much. One would think a new GM engine would me OK.

Oh the horror .don't send that motor back without pulling the pan and taking pics of counterweights on the crank and bottoms of pistons , head casting # and look through plug holes and see if you can see the piston tops ,dome shape ,oversize ....and please do tell ..

good luck with 800 runs new .i had 1 do the same thing i jumped threw hoops for them .the warranty he gives is useless .i know jk personally
and he is a clown .mine had the vibration also finally after months it spun 2 rod brgs .i called for warranty he said he needed the motor pulled apart .he guarateed it would have black on the inside of the pistons meaning detonation i told him it didnt then he said other brgs would be beat in the center.. they were all like new .so he had me drop the motor off at his shop ..end result after 2 weeks he called it detonation no warranty ..we rebuilt the motor had it balanced and put it back in the boat all on our dime ..now this motor had new oil cooler brand new carb from him and flywheel and exhaust manifolds ..good ol jk had every excuse .first timing then carb is to small then blamed exhaust manifolds ..now this job was done by me at the marina where i worked .. check the bbb on him many complaints ..i wish you luck and hope its something else because he wont help you

Im curious Smokin Gun, what crank was in your motor when you pulled it apart, GM forged steel or junk,

check harmonic balancer,should be able to feel the counter weight/bump out on the inside.those motors are supposed to be externally balanced.unless it got an aftermarket crank set up,,then all bets are off.as for "runs new" you get what you pay for.headaches

Thanks for all your help. John from 800 runs new gave me some things to try. He did say that the motor I bought has all GM forged internals and is spin tested before it ships. I plan to check for leaks, and try another distributor. We'll see....