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  1. #1
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    Help - intermittent noise

    Odd noise tonite while out on the lake. 1999 Mariner Optimax 150. Out running around at about 4000 RPM and starting getting a knocking noise from outboard. Back off and noise goes away, get back into e throttle and noise comes back. Back off to idle, let boat slow down, punch it and no noise, but then sometimes a few seconds and sometimes a few minutes before noise coming back. Always seems to be when on plane cruising around. Any thoughts? Doesn't seem to matter what RPM I am cruising at the noise eventually shows up. Could it be a clutch issue? Running a high five prop and no previous issues but have not had the boat very long.

    Thanks for any help.

  2. #2
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    I'd start by taking a real good look at the air compressor. Pull the belt off and spin it by hand. See if you have any play anywhere.
    '72 Glastron GT-150 Johnson Stinger 65
    '83 Hydrostream Varmint 2.4 225 Mercury
    '05 Hydrostream Voyager 250xs optimax

  3. #3
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    Quote Originally Posted by gregpro50 View Post
    I'd start by taking a real good look at the air compressor. Pull the belt off and spin it by hand. See if you have any play anywhere.

    DITTO.. how many hours are on the motor.. .they have a tendancy to fail after 300-500 HRS...

  4. #4
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    Try a different prop. Could be a harmonic imbalance between the prop/driveshaft/crank/flywheel. Have seen some give the symptoms you are describing because of certain SS props. The older inline 4 cyl's were notorius for it. Some of them you couldn't even run a SS prop. I have seen some Opti's do it too.
    Worth a look before you dig deeper. Changing a prop is easy just to check it.
    Also check the belt tensioner. They had a bad habit of breaking the boss of the block !

    b0atguy


    1986 Hydrostream Vegas XT
    Mercury Bridgeport EFI
    Brucato ACU
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  5. #5
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    I thought I would try a new prop tomorrow, but I will definitely check the compressor and the tensioner before I go out.

    Would any of these issues cause an intermittent noise?

  6. #6
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    Forgot to add local dealer scanned it when I bought and I was told 22hrs. Boat has been sitting for last 6 - 7 years.

  7. #7
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    does the motor jerk when the noise is present or only make noise?

  8. #8
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    It does not jerk, only makes noise.

  9. #9
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    Check to ensure that your flywheel is tight.

  10. #10
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    Looks like I wont be able to get out on the water today to try a different prop. I did check the air compressor, no bearing issues, no slop in the pulley. Check the belt tensioner- no problems there. Checked the flywheel, it's tight.
    One thing I did notice was that the intake for the air compressor was missing the grommet for the flywheel cover. Do the air compressors ever go into a unload state where air could exhaust out the intake casing the cover to thump up and down? Just a thought. Don't want to do too much arm chair diagnosing with out going and trying a different prop as b0atguy suggested.

  11. #11
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    had one doing same thing not a opti but wound up being the gearcase lost one tooth off the pinion very hard to diagnose just a random clunk or knock sound
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