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2004 - 40 HP Mercury 2 stroke

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  • 2004 - 40 HP Mercury 2 stroke

    I recently purchased a used 2004 40 HP Mercury 2 stroke engine. Engine starts and runs well, but it develops a rattling or knocking sound when I throttle down to get off of plane. I can't determine if the noise is coming from the power head or the lower unit. Is this normal with this engine?

  • #2
    Re: 2004 - 40 HP Mercury 2 stroke

    I have a 2002 that has made the same noise since new. Some people call it decel knock I think but it appears to be normal.


    • #3
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      Re: 2004 - 40 HP Mercury 2 stroke

      I have just purchased another boat with my first Mercury motor a 2004 40HP 2cycle. After many Yamaha and Evinrudes and was mortified to hear that knocking sound on decel, it does seem to be more of an engine knock rather than gearcase but is very difficult to pinpoint the source. I added seafoam as the boat had been sat, but a couple of tanks of 87oct fuel with sea foam added & the knock is still there
      Someone told me I was propped incorrectly and as I don't have a tach installed I cannot verify but it does seem to run good with plenty of top end and reasonable holeshot going to try higher octane gas next.

      Anyone else made any progress I D ing the source of the knocking ???