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Thread: What is this terrible knocking noise?

  1. #1
    Miss Fortune
    We just put a BRAND NEW Merc. motor in our boat. It starts right up, purrs like a kitten. However, when we accelerate, there's a terrible knocking sound. We haven't run any higher than about 3500-4000 RPMs since it's a new motor and we're breaking it in. When we tried to pull a tuber, the motor was cutting out or missing, and we were not able to pull a tuber or skier. The mechanic first thought it was a problem with a valve, possibly a "floating valve," but checked that out and there seems to be no problem with a valve. Any ideas of what it could be?

  2. #2
    In2Deep
    Have you checked all the vacuum lines, etc.?

  3. #3
    CHEETER-JOE
    Originally posted by Miss Fortune:
    We just put a BRAND NEW Merc. motor in our boat. It starts right up, purrs like a kitten. However, when we accelerate, there's a terrible knocking sound. We haven't run any higher than about 3500-4000 RPMs since it's a new motor and we're breaking it in. When we tried to pull a tuber, the motor was cutting out or missing, and we were not able to pull a tuber or skier. The mechanic first thought it was a problem with a valve, possibly a "floating valve," but checked that out and there seems to be no problem with a valve. Any ideas of what it could be?
    Maybe fouled plugs?? Altitude?? I may be wrong, but it is only happening when the motor is under a load.

  4. #4
    502procharger
    replace your plugs, i just went through something a little like this, plugs were less that 1 hr old. i put new ones in, made sure they were all gapped, away she went

  5. #5
    Miss Fortune
    The plugs were already replaced. 502procharger, were you getting a terrible knocking or popping noise also?

  6. #6
    DEEZ NUTTS
    Popping noises, no power, did your mechanic check to see if a valve is not opening? How does the oil look? Might be another toasted cam to add to the list that these boards hold.

  7. #7
    502procharger
    i was getting a popping spitting and backfire's under load, but i did not get a knock. good luck

  8. #8
    Costello
    My guess is valve float, that's why the popping is happening as well.

  9. #9
    Seth Livzz
    The RPM the problems are happening are pretty low for the problem to be valve float.
    Is the sound more of a knock or a ping?
    Have you tried using a scope to see if you can isolate the area you hear the sound coming from?
    Is the fuel fresh?
    Might wanna check the timing as it could be detonating. Detonation makes some horrendous sounds.

  10. #10
    shockwavebd
    are you getting a black transom

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