Vibration (not the prop)

[Wakemaker]

2000 Tige 21V Limited, Mercruiser 350 Magnum MPI. Getting vibration that rumbles through the hull while at idle speed. Not huge but definetly was not there beginning of the season. Happens at idle speed and also is evident when transmission is not engaged and I rev the engine at low speed but goes away once past that (prop not engaged when checking it this way, which is why I say not the prop). No visible shaking of the engine by the naked eye. Any obvious or not obvious reasons you can think of?

[dogbert]

2000 Tige 21V Limited, Mercruiser 350 Magnum MPI. Getting vibration that rumbles through the hull while at idle speed. Not huge but definetly was not there beginning of the season. Happens at idle speed and also is evident when transmission is not engaged and I rev the engine at low speed but goes away once past that (prop not engaged when checking it this way, which is why I say not the prop). No visible shaking of the engine by the naked eye. Any obvious or not obvious reasons you can think of?

Assuming what you're describing is a rough idling engine, I'd check the following:

1. Distributor Cap (may need replacing)
2. Spark Plugs (one may be misfiring)
3. Spark Plug Wires

[chpthril]

I gatta 2nd Dog on the engine miss. Also look at the PCV hose. A vacuum leak will cause a lean miss at low engine speed, but go away at higher eng speed.

[Wakemaker]

I will check out the rotor cap and wires. Question, are these marine specific if I need to replace or can I get em local?
Any other ideas out there? Tks

[spharis]

ignition protected is recommended, in case there is a spark.......BOOM!

[FIC]

If its not a missfire, I have had problems with the flex plate damper springs starting to come apart and eventually tearing the center hub out. This usually occurs due to throwing the boat into reveres at to high of an rpm.

[dogbert]

ignition protected is recommended, in case there is a spark.......BOOM!

I'll second this suggestion.

[chpthril]

Here is your boat

[chpthril]

Here is your boat on automotive parts!

[dogbert]

I have questions

[lee]

2000 Tige 21V Limited, Mercruiser 350 Magnum MPI. Getting vibration that rumbles through the hull while at idle speed.

Is it in the 1100-1300 rpm range?

[R&T Babich]

Lee,
Our 2000 21V Riders with 350 MAG MPI, 1.46 Walter V-drive and OJ 14 x 18 also seems to have a low vibration in the rpm range you mentioned 1100 -1300. It's smooth at idle, then seems to have a rumbling in the hull off idle up to around 1500-1600, then smoothes out. It takes off after that and quickly burys the 35 MPH speedo so I don't really know how fast we're going. Above 30 MPH it sometimes starts to porpoise which I can take out by adjusting the TAPS setting. The engine seems quite smooth and the prop visibly looks fine, only 45 hours on the boat. It's almost as if we were lugging the motor because the "gearing" is too high. (My '77 E150 van with factory 2.75 gears ran much better with 3.25s in it.) Why did you ask about that rpm range?

[lee]

A GM 5.7 engine has a Harmonic Resonance to it in 1100-1300 rpm range. Most boats will have a vibration in that rpm range due to the engines rotation.

[Wakemaker]

Tachometer on the fritz so I cant tell you the specific rpm. It happens with or without the transmission engaged, but for description I can tell you its most apparent in the range where I come out of neutral up until about 5-7mph. Idling around the lake isn't the quiet purring sound it used to be, now I need to speed up to reduce the rumble. Any input appreciated

[Wake Havasu]

Are your Engine mounts OK?