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New Mercury 40 FourStroke Rattling/Screeching

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  • New Mercury 40 FourStroke Rattling/Screeching

    Hello. I just bought a new pontoon with a new Mercury FourStroke 40 motor. When in gear between 1700-2200 RPMs, there is a rattling/screeching sound that noticeably disappears when outside this range. It doesn’t happen in neutral and also doesn’t happen when trimmed up significantly.

    Video: https://youtu.be/yEyEFGSXWfo

    Doesn’t appear to be the cover or latch since it happens even with the cover off and when ensuring the latch is secure.

    I can contact the dealer, but wanted to know if anyone has a
    quick fix for this issue. It doesn’t appear to have an impact on performance, but is certainly distracting and annoying. Shouldn’t be happening on a new motor!


  • #2
    While searching for the source of the sound, keep in mind that it may be coming from something near the engine rather than the engine. Every object has a natural vibration frequency and that vibration will increase when it is being stimulated at that "resonant frequency." It is likely that when your motor in in the 2000 RPM +/- range its vibration is stimulating something in or near the motor.

    If you can safely reach back with a boat hook or broom handle to touch areas around the motor while the noise is present, you may find the spot it is coming from.

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    • #3
      Originally posted by unirocket View Post
      Shouldn’t be happening on a new motor!
      That's why it should go back to the dealer.
      BOAT SPECS | FORUM HELP | STARCRAFT FORUM | SHOP iBoats
      Please, no PM's (Private Messages) regarding boat/engine problems.
      That is what the forums are for.
      Only forum/moderator issues will be answered in PM's.

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      • #4
        This is subjective as all get out, but I would never call what I am hearing in that video a rattle or screeching. I would call it harmonics or a resonance. It happens. It's a vibration set up by the motor running in a particular rpm range.

        Too, I would not get in too big a hurry to call it a dealer responsibility or defect in the motor. I WOULD ask the dealer to give you a hand - if they are on the water. If it's a "dry" dealer, maybe they could spare a tech long enough for him to go for a ride with you?

        Myself, I might be tempted to try a different pitched prop. Maybe one based on prop tuning with a target max rpm of about 5500. There's a chance that the prop itself could be causing the resonance.

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        • #5
          Originally posted by ahicks View Post
          This is subjective as all get out, but I would never call what I am hearing in that video a rattle or screeching. I would call it harmonics or a resonance. It happens. It's a vibration set up by the motor running in a particular rpm range.

          Too, I would not get in too big a hurry to call it a dealer responsibility or defect in the motor. I WOULD ask the dealer to give you a hand - if they are on the water. If it's a "dry" dealer, maybe they could spare a tech long enough for him to go for a ride with you?

          Myself, I might be tempted to try a different pitched prop. Maybe one based on prop tuning with a target max rpm of about 5500. There's a chance that the prop itself could be causing the resonance.
          Thank you! I’m happy to report that I replaced a 3 blade prop with a 4 blade and slightly different pitch l, and that resolved the issue.

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          • #6
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