Rough/Vibration at 1200 rpm 20V

[paulct]

I've had a problem with rough running between about 1200 and 1500 rpm when in neutral. you can really feel the boat shake. This is a 2007 20V. Smooths out over that range and below.

I've had the dealer look at it twice, and they claim to see no problem. Figures.

It almost sounds like its missing on one cylinder. Boat only has about 30 hours. I *know* the dealer should fix it, but here is what I plan to try, would appreciate any other suggestions:
- Replace the spark plugs (never done, with 30 hours and platinum plugs, they should be ok?). See if any look funny.
- Replace ignition wires.

Any other obvious simple things to do?

Thanks.

[dingleberry]

Does the engine sound rough, or do you just feel vibration? If vibration is the only symptom, check the prop and the drive shaft carefully.

That's all I've got. Hopefully others will chime in with better suggestions.

[dogbert]

What motor do you have?

It's unlikely spark plugs need to be replaced with only 30 hours on the boat. My dealer recommended they get changed every 400 hours along with the spark plug wires.

If you were missing a cylinder, you'd get vibration across more than just that range.

One other thing to check would be to make sure the engine mounts are aligned properly. Is it possible that they one of the mounts is loose?

Once we know what engine you have, we provide some more suggestions.

[chpthril]

I wouldnt just start tossing parts at it yet. You may have a bad plug or bad wire, but they all dont need to be replaced with only 30hrs.

Sounds to me like it may be the normal harmonic vibration of a V8. 1200rpms is about right.

[paulct]

Oh, sorry I forgot to mention it was the 5.7 Marine Power 340hp. It is really most noticeable in neutral, less so underway. So I don't think it is shaft/prop stuff.

I've had a lot of V8's and never really noticed this before. I'll check the motor mounts too.

I just figured it could be a bad/intermittent plug or wire, and as long as I am checking, might as well pop new ones in, and if they are all perfect, I have spares when the time comes!

Thanks,

Paul

[chpthril]

If a boat, like cars, has big rubber mounts, then the natural harmonics are never noticed. I would re-torque the drive-train mounts and see what happens. It's not impossible for a plug or wire to fail, just too early for them all to need replacing, especially when the plugs are like $8 a piece.

[GIZMO]

I Think chp is correct; this motor has a natural vibration at that rpm. It will re-occur at 2400rpm, 4800rpm, ect. But the frequency gets higher and less noticeable. Why are you running the boat at 1200rpm in neutral? Many new car’s come with “active mounts” to reduce it before its transferred into the cabin. In neutral you are not rotating a lot components that contribute to the rotating mass and lessen vibrations.

[paulct]

I replaced the plugs today, just for fun. Whoever at marine power designed that exhaust manifold should have to change the front plugs! Thank goodness I had a thin walled plug socket that just made it on. There was no evidence of plug problems, other than they did not look as good as I would have thought for 35 hours. I should have taken a close up shot, but forgot.

Anyway, the roughness seems about the same, maybe a tad better. The vibration happens underway as well as in neutral, by the way, at just about 1400. It really is not *that* bad, just noticeable.

I don't normally run up the motor in neutral, but I have found that when launching off the trailer, after a cold start, it won't back off the trailer without stalling. So I run it up a bit to get it warmed up, then it won't stall when going into reverse.