I had new carb'd 502 long blocks installed this winter. During water test this weekend I noticed a significant vibration 3000 RPM's and above, it phases in and out (it comes and goes), definitely not the norm. I can feel the vibration in the throttles, the steering wheel and through my feet on the floor. The 502's don't sound "synch'd and phased" like the old droning 454's I had either.

This winter I also had HP gimbals installed with new gimbal rings, new engine couplers. The engines are definitely over-carbed, Holley 850's which have to be rejetted or adjusted somehow because they're spitting back up into the flame arrestor if I increase throttle too quickly.

Are these somehow related or separate issues? Where to begin troubleshooting the vibration issue? I don't want to damage anything. The mechanic hasn't had a chance yet to start looking for the problem.

Start by isolating the vibration. Pull the drive off and run the engine. Does it still do it? If so, then you need to start looking closer at the engine. Make sure that the firing order is set correctly. Double check this as it's really easy to make the same mistake twice. I guess the next thing would be to pull the engine and make sure the flywheel in on correctly. Mine was off by 180*. The engine would idle just fine, but when the rpms would come up to around 3k the whole boat would shake. Somebody, for some reason, had cut the dowel pin off the crank. You had to look really close at the crank as it was almost impossible to see where it used to be.

Do you know for a fact that they alligned the drive? Also one more thing that is unlikely but I saw it happen once before is the key in the harmonic ballancer could have broken off and allowed the ballancer to turn a little, or a wrong balancer or something like that. What is the history on these engines? Did you rebuild them or are they take outs from some one else?

Jon

I'm going to the shop today to discuss the plan to troubleshoot this thing, will pass along the thoughts. The only time I can feel the vibration is while water testing, don't recall feeling it when it was on the trailer.

The engines are brand new 502/450HP long blocks from Merc. Took all the bolt-on stuff from my old 454 Magnums, but supposedly they installed a new engine coupler. I'll find out what he thinks later today and let you know. Thanks a million for the quick replies.

One more thing to check is the runout of the coupler. I always make a shaft that barely fits the pilot bore of the coupler splines and then set up a dial indicator to the shaft and turn the motor over by hand. I have seen some out as much as .025. Dont know if that would cause that kind of vibration though but I know it doesnt do the universals any good. When you check the alignment, alway rotate the motor 90 and 180 degrees and re-check it. That vibration you are having sounds more like a motor issue to me though.

Are you absolutely sure the props are OK?

1-Bad prop?

2-When motor back fires when dumping the throttle it is because the air/fuel mixture is 2 lean. Jetting may be too lean or the accelerator pump may be bad or adjusted wrong.

Are the flywheels interchangable between a 454 and a 502 ? If not and they replaced just the long blocks and used your old 454 stuff that could be the problem.

Also Turbojack is right on with the carb thing.
Generally a pop thru the carb is lean and if it stumbles under a load it is rich.

Just got back, they're coming up next week to troubleshoot. The vibration happens with 2 sets of props. My old 3 blades from last season which I know are fine, and a new pair of 4 blades from a buddies boat. They're also going to start with the simple stuff, recheck the alignment, firing order, etc. Along with a new engine coupler, the flywheels were also supposed to be new and included with the long blocks.

The carb problem was confirmed, they told me they plan to rejet them.

I'll follow up when we finally get to the bottom of it and let you know what it was for sure. Thanks again for all the replies to this point.

Make sure the flywheels are correct for the new engines.
The 502 and 454 flywheels are different.
Did they come with the new engines?

I agree it sounds like the flywheels and balancers, but think the vibration would be bad at all rpms and then get worse. I installed a wrong flywheel on an engine once and it nearly shook apart just starting it.

If you have Dennis Moores book it explains the differences between mark IV, gen V and IV balancing, and between cast and forged rotating assemblies. They all take different part numbers. Let me know if you want me to look it up to be sure.

BT