Thumping Noise upon Starting Merc 140HP

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Staush

Cadet
Joined
Aug 17, 2006
Messages
6
I'm new at this so I need advice. When I first start the engine it makes a thumping noise 5 or 6 times like the motor is vibrating against the hull. Is this normal or maybe a loose motor mount? Its a 1975 model and I have not idea if they were ever changed.
 
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paulie0735

Chief Petty Officer
Joined
Dec 6, 2005
Messages
463
Re: Thumping Noise upon Starting Merc 140HP

Is this a new boat for you, has this just started to happen? Was it ok previously? Does the noise go away after the "5 or 6 times"? Can you feel the knock in the hull itself or just hear it? If you shut down and restart does it knock again? Or is this just when it’s cold? Lots of questions and lots of answers needed to narrow this down a little.
 
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Staush

Cadet
Joined
Aug 17, 2006
Messages
6
Re: Thumping Noise upon Starting Merc 140HP

This boat is new to me. The noise only lasts a minute and then stops. I can't feel it through the floor and I haven't had a chance to have someone else start the motor while I tried to pinpoint the location.
 
Don S

Don S

Honorary Moderator Emeritus
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Messages
62,321
Re: Thumping Noise upon Starting Merc 140HP

Without actually hearing what you are hearing, it's going to be very difficult to diagnose. It may be water in the cylinders and causing a partial hydrolock. This can also completely stop the cranking of the engine, or make it stop and then start cranking again, but with a noise.
Might want to pull the plugs and check for water in any of the cylinders.
 
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DHPMARINE

Captain
Joined
Dec 16, 2003
Messages
3,688
Re: Thumping Noise upon Starting Merc 140HP

Just an odd thought since the boat is new to you.Is the drive down when you start it? It should be.

DHP
 
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Speakrdude

Ensign
Joined
Feb 25, 2004
Messages
942
Re: Thumping Noise upon Starting Merc 140HP

I had a similar problem only when the boat was in nuetral. Turned out to be a dead cylinder due to a bent (warped) valve causing a no-compression problem in that cylinder.

Yours could be a simple misfire, bad plug, plug wire, injector, etc...
 
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paulie0735

Chief Petty Officer
Joined
Dec 6, 2005
Messages
463
Re: Thumping Noise upon Starting Merc 140HP

Could also be something not right with the drive, have you had the boat on the water yet? Seems odd that it will knock loud enough that it concerns you than the knocking stops!! I'm thinking oil pressure but not convinced yet. Are you sure it’s in neutral when you start it? You're really gunna need to get back there and listen carefully while somebody else starts the boat.
 
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Staush

Cadet
Joined
Aug 17, 2006
Messages
6
Re: Thumping Noise upon Starting Merc 140HP

Thanks for the replies. I haven't had it in the water yet, but I'm leaning toward a misfire or something that causes a vibration. Like someone said I need to closely listen while someone else starts it. Thanks again.
 
Don S

Don S

Honorary Moderator Emeritus
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Aug 31, 2004
Messages
62,321
Re: Thumping Noise upon Starting Merc 140HP

Like someone said I need to closely listen while someone else starts it.
Click to expand...

No, you don't need to listen, you need to do some LOOKING before you destroy something expensive.
Pull the plugs like I mentioned earlier. If you are getting water in there, you can bend rods, destroy starters, take the teeth off the flywheel.
Could be the ujoints, pull the flippin' drive and look.
Then with the drive off and the spark plugs out turn the engine over and see what happens. Watch the engine when someone is cranking it over and see what is happening..
Don't just assume everything will be fine, assume it's all going bad and try to make sure you don't make it worse.

And if you don't have any help, look at this thread.

http://forums.iboats.com/bbBoard.cgi?a=viewthread;fid=24;gtid=1233493
 
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