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  1. #1
    Senior Member
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    Dec 2003
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    Austin, Texas
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    My motor is knocking when slowing down??

    I have been noticing the last few times out, a noise or knocking sound, when I slow down. My 1995 Mercury 115hp outboard, starts and runs just fine. However, when I slow down from from 3500rpms plus, down to a halt or 1000 rpms, that there is a knocking sound, almost like a piston is not getting oil or burned or ??

    My boat has a oil reservoir on the engine, pumping oil into the cylinders. It was replaced about 4 years ago, due to a alarm that kept going off.

    Maybe I should just pour oil right into the gas tank??? I am not certain to what the oil to gas ratio is on this motor....I have always questioned oil injector engines...

    Anybody heard this before? I might have to take to a boat mechanic, before I go back out. I do not know if my one of my four pistons is making this noise or not...

    I hate to take it out, thinking I can blow an engine. That would not be a good Christmas present....

  2. #2
    Administrator
    Join Date
    Oct 2006
    Posts
    1,013

    Re: Possible boat motor problem??

    Hard to tell without hearing the noise in person. From your description it could be piston slap. The main components that guide the piston are the piston rings and crankshaft bearings. Have you de-carbed the engine recently? This would help free up the piston rings if they are gummed up with carbon. I'm hesitant to think the rings are the source of the knocking. Most likely either crankshaft bearings starting to give out or good old engine knocking due to misfire/pre-detonation of combustion. This can happen if spark timing is off due to change in rpms. Do you have an electronic ignition control (ECU) on that motor? I'd be glad to listen to it next time we'e on the water.

    Keep in mind most of my engine work is on autos. Boat motors are similar in concept, but a totally different beast. Swimbait and EA have great boat engine mechanics if you want to get it checked out for good measure.

    Hope everything is alright.

    -Rich


  3. #3
    Senior Member
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    Dec 2003
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    Austin, Texas
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    Re: My motor is knocking when slowing down??

    I replace the spark plugs and drain the lower unit oil...But never a " Decarb" on the engine. Is it something I can try?

    ONE OTHER NOTE:
    It runs well. The only thing that seems odd is after I have had the engine at higher speeds and I decelerate quickly the engine makes a metalic knocking noise that sound like there is something loose in the engine. Or maybe more like knocking cylinders. It is not noticable unless you are listening for it. Although I did see my friend look back at the engine when it happened once.

  4. #4
    Senior Member
    Join Date
    Mar 2007
    Posts
    597

    Re: My motor is knocking when slowing down??

    If it while slowing down it is more than likely the lower unit. Check your oil and see if it has debris in it (metal). It will resonate and sound like the motor.

    If not that probably a broken reed, and is hitting the cage.


    Other than that I don't know, but most motors usually blow right before they run real well. So how was it running ;D

  5. #5
    Senior Member
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    Re: My motor is knocking when slowing down??


 

 

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