1981 Mercruiser 140 Vibrates entire boat at idle

[chris0061]

Friend of mine picked up a 1981 Mark twain with the mercruiser 140 in it. We started it up last Sat. and the boat was vibrating to beet heck at idle, Thought maybe something wasn't aligned so took the out-drive off, started it and did the same thing, took my alignment tool and perfect alignment. We took it to the river and it seemed to even out at about half throttle, wot was good at 4300 rpm with plenty of power and not so much vibration. He is buying a tune up kit this week and were going to try that first. Also the carb seems a little sluggish at times. Just throwing that in for ideas. Even on the muffs in the drive way you give it a little gas and it wants to even out some.

 

[Bt Doctur]

Re: 1981 Mercruiser 140 Vibrates entire boat at idle

It should not vibrate at all with the drive removed. A weak /dead cylinder will cause it to vibrate.
Do a compression test first . Also check #4 spark plug to see if it looks a lot whiter than the rest. could be a sign of water ingestion due to a bad/leaky riser.

 

[chris0061]

Re: 1981 Mercruiser 140 Vibrates entire boat at idle

Is the number 4 cylinder towards the front of the boat or the stern? I pulled all the plugs but didn't have my compression tester with me and the most forward plug looked whiter than the other three.

 

[chris0061]

Re: 1981 Mercruiser 140 Vibrates entire boat at idle

also the boat had all kinds of power on hole shot and going to wot.

 

[Bt Doctur]

Re: 1981 Mercruiser 140 Vibrates entire boat at idle

not sure if this is for your but fairley close, has a lot of useful info in it.
http://www.boatfix.com/merc/Servmanl/13/13covr2.pdf

 

[chris0061]

Re: 1981 Mercruiser 140 Vibrates entire boat at idle

Did'nt get it all Doc

 

[Bt Doctur]

Re: 1981 Mercruiser 140 Vibrates entire boat at idle

link does work, click on the center box to get to the index

 

[stonyloam]

Re: 1981 Mercruiser 140 Vibrates entire boat at idle

Agree with the Dr. Sounds like a bad cylinder. A quick check till you can do the compression check. Get it running and CAREFULLY pull the plug wires one at a time, to see if one cyl does not make a difference. If one is not firing, try swapping the wire with the one from another cyl, and then the plugs. If all of that does not make a difference, your compression test should tell you what is wrong. BTY #1 is forward.

 

[chris0061]

Re: 1981 Mercruiser 140 Vibrates entire boat at idle

I just happen to pull all the plug wires one at a time and there was noticeable roughness when I pulled each one. When I go over to work on it sat, I'm bringing my dwell meeter, compression tester, timing light. I'm really hoping that just a complete tune up will solve it but will definitely start with a compression test and go from there.

 

[stonyloam]

Re: 1981 Mercruiser 140 Vibrates entire boat at idle

noticeable roughness when I pulled each one.

That is always a good sign. Good luck

 

[Fireman431]

Re: 1981 Mercruiser 140 Vibrates entire boat at idle

Bad plug, bad cylinder, bad plug wire, cracked distributer cap, crossed plug wires.

 

[Winsok]

Re: 1981 Mercruiser 140 Vibrates entire boat at idle

old post i know but im curious if you ever solved the issue, i have an old 71 mercruiser 140, and it vibrates alot at idle too, my tach reads about 1000rpm at idle witch might be a little high, but once i get cruseing it is nice get about 40mph on the water no problem.

 

[dollarten]

Re: 1981 Mercruiser 140 Vibrates entire boat at idle

old post i know but im curious if you ever solved the issue, i have an old 71 mercruiser 140, and it vibrates alot at idle too, my tach reads about 1000rpm at idle witch might be a little high, but once i get cruseing it is nice get about 40mph on the water no problem.

I do remember these engines being a little rough in the idle department . It's like you can feel every cylinder fire when it's idling Sounds like Winsoc is experiencing the same vibes with his engine If it gets up and goes right away , I would not worry The 140 is a 181 cubic inch engine and does have a rougher idle as compared to the 120 , that being a 151 cubic inch engine Raise the idle a bit in order to smooth things out but not too high that you will bang it gear