Diesel vibrates at low rpms.

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weinie

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Sep 6, 2010
1,297
Jeanneau 349 port washington, ny
Over the winter, I had the stuffing box, cutlass bearing, prop shaft and coupling replaced.

However, it seems I am getting very strong vibrations when idling in neutral or in gear. My idle speed is set to about 1200rpm (confirmed with a laser tach).

The vibration completely goes away at higher rpms and the boat really flies under full power without any vibration at all.

My mechanic verified the alignment and said it was spot on. Motor mounts are original but look ok.

Any ideas?

ETA: Just restating: this problem occurs in gear AND IN NEUTRAL.

ETA2: Engine is Universal M18 on a 87 Catalina 27
 
Feb 26, 2009
601
Oday 30 Anchor Yacht Club, Bristol PA
2 cylinder? they all rock at low rpms from what others tell me I know mine does
 

bria46

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Jan 15, 2011
286
Oday 272 Waukegan, IL, Sarasota, FL
Even though your mechanic said that your Motor Mounts looked OK. It's very tough to check the internal parts of the mounts.
 
May 24, 2004
6,838
CC 30 South Florida
Alingment and components balance should not affect engine vibration while in neutral. Check the motor mounts, just them looking good is not enough. One thing you can do is accelerate and decelerate the engine a few times in neutral while observing the engine and motor mounts, any marked roll of the engine from side to side could point to a weak or broken motor mount. At 1200 RPM which could be considered a fast idle you should have very little vibration. It is the function of the motor mounts to tame out any engine operating vibrations. As the engines age and components wear down we experienced increased vibrations and it is at this time when new motor mounts can be most effective.
 
Apr 8, 2010
1,637
Ericson Yachts Olson 34 28400 Portland OR
The rubber compound in the old mounts in an '87 boat is long past its replacement date.
It isn't just the engine hours, it's the years...

Put in a new set and do a true alignment from the center of the cutlass bearing all the way forward. Be sure that the shaft is centered in the shaft alley.

We have done all that and the difference in (and lack of) vibration when complete is very noticeable.

LB
 

weinie

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Sep 6, 2010
1,297
Jeanneau 349 port washington, ny
Ok,
I ordered 4 new mounts and will try to install them myself. I've read maine's article on doing this with a scissor jack.
Anyone else try this themselves?
Anywhoo, I hope this cures the vibration and its not something else wrong.
 
Sep 25, 2008
615
Morgan 415 Out Island Rogersville, AL
I used a scissor jack with good luck. Some people use a kid's jump ball. The cram it under the oil pan and inflate it.
 

weinie

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Sep 6, 2010
1,297
Jeanneau 349 port washington, ny
Right now it feels like its gonna vibrate right off the mounts though.
 
Jan 22, 2008
319
Hunter 29.5 Gloucester, VA
My 2GM20F did (does) the same thing. Around 850 or so it bounced around like it wanted to come out of the hole. I replaced the mounts and it was the same thing. I bump it up to around 900 and it's much improved. Like you full throttle is pretty good. Only seems like the harmonic vibrations. I haven't checked the calibration with a tach yet but my 850 on the gauge seems like a reeeaal slow 850. Right now mine is set at 900 on the rpm gauge and all seems fine. shifting is smooth and vibrations are significantly reduced
 
Feb 26, 2009
601
Oday 30 Anchor Yacht Club, Bristol PA
It was explained to me that the more cylinders the smoother the engine will idle. Like 6 and 8 cyl car engines. small 1, 2, cyl engines do need "rock" at certain rpms. Dave, I think it is some sort of harmonic!
 

weinie

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Sep 6, 2010
1,297
Jeanneau 349 port washington, ny
Dave and Denise,

You guys may be right however, I'm thinking that since I did not have this problem last year I need to look at what has changed. What I'm thinking is that after the mechanic re-aligned the engine after the shaft and coupling change over the winter, the new position of the engine on the mounts may be relying more on the worst of the four mounts (the forward/strdbrd one) than it had previously for dampening.

And frankly, I don't know what else it could be. As others have said above, the idle speed is already a tad on the high side. The engine purrs at high speed and with my newly scraped bottom, my 27 footer was doing 6.8knots in calm water last week!

I have to replace my muffler which was leaking, so as soon as I do that this week, I will try to post a video.
 

weinie

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Sep 6, 2010
1,297
Jeanneau 349 port washington, ny
My co-worker just suggested I put the stray cat thats been meowing all night and keeping me up under the engine to absorb the vibration.:eek:
 
Jan 22, 2008
319
Hunter 29.5 Gloucester, VA
I'll see your cat and raise you two dachshunds that won't stop barking. :cussing:
 

weinie

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Sep 6, 2010
1,297
Jeanneau 349 port washington, ny
Funny you should say that. Last year we put down 2 dachshunds that my wife and I had for almost 15 years of our lives. They were cute as hell but they drove me insane. If they were here today, I think each one would fit perfectly under each side of the engine. Good thing wife unit didn't read sailing forums ;)
 

gpd955

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Feb 22, 2006
1,164
Catalina 310 Cape May, NJ
Weinie,

I had an annoying vibration at low RPM on my Universal M25 in the Catalina 310 and after searching for weeks found that it was simply the wood cover in the quarter berth was in contact with the waterlift muffler and 1200 was the exact RPM needed to cause vibration. Once we hit about 1600 it stopped. I went crazy trying to figure it out and I only found it when I was at wits end and sat down in the berth, causing the vibration to stop.

Not saying that this is your problem or if your boat is even laid out that way but if it is a similar issue, it may save you some $$$.

I have no vibration at all now!!
 
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