Vibration (not the prop)

[Wakemaker]

Mounts look good and no visible vibration. I am going to check out the suggestions on the ignition system and will post the results. Tks

[R&T Babich]

This may be our vibration problem. We bought our 2000 21V a couple of months ago. It has the 315 hp Mag MPI and we've only had it out a couple of times. Compared to our old boat there's quite a bit more power. I pulled the plugs to run a compression test. I'm sure I didn't crack this one taking it out so I think this may be our vibration. The plugs are the factory originals with paint overspray on them. Now I can't wait 'til spring to take it out and see what kind of power it really has.

[imscarlet]

Can't quite see the tip in the photo but how does it compare colour wise to the other plugs. If it is a blacker colour than the others it will most likely be the source of your prob.

[chpthril]

I think he's is refering to the crack in the insulation.

Unfortunately the crack looks clean (no black carbon tracking), so that plug may have cracked during removal. Also check the boot that was on that plug. If that crack is the source of the misfire, then the sparkplug boot will need replacing due to carbon tracking as the 'spark' will run right down that trail of carbon to the cylinder head.

[spenchey2]

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?

I had this same problem i took it back to the marina and the tech looked at and drove it with me and he determined that the exhausted was hitting on the transom makeing the noise. They cut the exhaust back, farther away from the transom and it worked.

[R&T Babich]

Update: The rest of the plugs look like the one on the left. The cracked one is on the right. The cracks go down into the metal case and the whole ceramic top wobbles a tiny bit. I was wondering why no arc trail, too, and the boot was the first thing I looked at (looks fine). After looking at where the cracks go I think any arcing would have been inside the metal case. The boat only has 48 hrs on it and the compression test was the best I've ever had - highest 190 lbs, lowest 180 lbs.

Sort of related: I was helping a relative troubleshoot a 454 Chevy that wouldn't run, just sputtered. It had an intermittent, but strong spark. Three people inspected the cap and could see nothing wrong. We changed everything else, rotor,plugs,wires,module - no luck. We finally put on another cap and the problem was gone. The old cap was arcing internally, so no fixed arc trail. Put the bad cap back on and the problem returned. Lesson - a cap can look perfect, but try another one anyway.

Another sort of related: A neighbor was driving to the store in an older V8 and on the way home the engine started making a sort of hissing noise. The ceramic part on one of the spark plugs had blown out of the metal casing and was hanging by the plug cable.

[imscarlet]

The one on the left has been working as the end is a nice colour whereas the one on the right is blacker because it has not been burning the fuel

[Wakemaker]

I had this same problem i took it back to the marina and the tech looked at and drove it with me and he determined that the exhausted was hitting on the transom makeing the noise. They cut the exhaust back, farther away from the transom and it worked.

This sounds a lot like what I experienced. Can you give any more detail on what you mean by cutting the exhaust farther away?????????? Tks

[spharis]

Nice find.....You should have seen how much fuel was on mine from having short thread plugs installed; soaking wet, and smelled like fuel.