Hey everyone.......newb boater here. Long time hot rodder, ran several Sportsman/Alcohol cars when I lived in Indianapolis + my 1969 Pro Street 572 Chevelle, ZR-1 Corvette, couple of motorcycles, etc.

This looks to be the best boating forum on the net from what I can see.

Ayway, as I am getting into my mid 40's, I bought a nice 1990 Cobalt 243 back in February (no test drive since I live near Chicago) for some relaxation and yet still have some performance. The original owner re-powered a few years ago with Merc 454 Mag and a new Bravo-1 outdrive......boat has been meticulously taken care of from all appearances. Probably a mistake to buy it without hearing it run on the water.....oh well it looks good anyway.

I finally had some time this past week to get it out on the water. Boat runs strong except for a significant vibration above 2k rpm. The vibration is not normal, so I did not run it for very long.

I bought all of the Mercruiser tools for drive alignment, bellows r&r, gimbal bearing r&r, various seal installers, spanner wrenches, etc... and pulled the outdrive today to check for worn u joints, mis-alignment, or worn gimbal bearing.

The u joints, drive shaft, bearing, bellows, etc....look new...no water intrusion, rust, or anything amiss. Gimbal bearing and u joints are smooth and tight.

I rigged up a water fitting to run the engine on the trailer without the drive in order to isolate and troubleshoot. The vibration is still the same without the drive attached. The only thing left is the coupler. The splines on the coupler look well greased and are square with no signs of excessive wear.

Any Ideas?

Thanks!!

good luck with that,im getting ready to pull motor out tomarrow same prob sounds like one of the mounting bolts came loose and came out it ratteld around last weekend now its stuck somewhere in the bell hsg and the coupler is is hitting it 3 times per revolution sounds very bad .hope its not to bad after i get it out and look at it

if drive is not mounted, i don't think it is coupler, unless it is so loose it is flapping in the breeze. shine a flashlight in thru gimball bearing while somebody cranks engine on starter. any severe runout may be visible. but it is possible somebody put internal balanced (neutral balance) flywheel on the 454. especially if it was up-gunned from a 350. i had a 400 small block and somebody sold me 454 flywheel said it was 400. externally balanced, but done differently than 400. boat vibrated bad at about 2000. rub rails shook like the front forks on my old triumph at idle...

I suspected as much Derek.......the boat originally had a mk IV 454/330 hp and it was re-powered with a gen v 454 Mag (forged internals). The original flywheel was likely re-used.

The flywheel balance is not the same between mkIV and mk V/VI engines. Looks like the motor has to be pulled and the correct flywheel installed. What a PITA for something so simple.

Another good example of why one should develop good mechanical skills and experience. I have had numerous problems in the past few years with new cars (under warranty), motorcycles, and now a boat where the mechanic that did the work slapped it together, charged a sh!tload of $$$ and sent it out the door with no attention paid to detail whatsoever. These Cobalts are nice boats....the hack job that was done to the wiring for the re-power was a sorry sight.

The guy I bought it from has the receipt for the re-power from 2010......paid almost $10K......I guess I will call the shop on Monday and see if they will do anything. I won't hold my breath.

Since I will likely have to pull the motor myself, what do you guys recommend? I have r&r'd dozens of engines from cars, but, any tips on Mercruisers would be greatly appreciated.

Since I already have the drive out it appears that drain fluids, disconnect hoses and wiring, unbolt engine mounts, lift motor out.

I have a shop crane for normal engines, but, it will not reach high enough.

mercwizard,where are you located?

Bloomington

so, you and mike are about 2 hours apart. if stuck doing it at home with your equipment, i scrounge small pallets, hang cherry picker over motor, drop come-along from cherry picker. get it as high as possible and stack pallets or 4x4's or something underneath. you don't need it all the way out to pull flywheel. of course i do this with itty bitty small blocks...most marinas will pull an unhooked motor for 50 bucks or so...or find a place with a dead travel lift and hang come-along from the straps.

MercWizard,
You are looking for the casting numbers 805 on the flywheel. If it doesn't have those numbers, it's the wrong one. I think the old MK IV used a flywheel with 457 casting #. Not positive on that though.
Eddie

my old 454 flywheel has 457n as last 3 numbers. i guess it is the fancy one with a higher nodularity content. i got flywheel in 91 or 92 and it was junkyard part then.