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Thread: Engine knock at low RPM

  1. #1
    Registered User bmweuro's Avatar
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    Engine knock at low RPM

    I have a 76 R90 motor that has new rod bearing and the small end bushings have been replaced and sized to the wrist pins. The motor has 40mm Bings with 170 main jets, 45 idle' 2.68 needle jets and I'm running W5DC plugs and the look perfect on a crank fire ignition with 26 degrease full advance. The noise sounds like it is coming from the center of the motor. The main bearings have about 30K or so. Oil pressure is between 30 and 80 pounds. The motor sat for 10 years. The noise happens at low to mid RPM under load only while lightly accelerating above 170 degrease.

    Any ideas on what the noise might be?
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  2. #2
    Registered User bmweuro's Avatar
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    Any idea's?
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  3. #3
    Administrator 20774's Avatar
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    Not sure if this applies, but I recall reading something about a staking pin that can come loose from the lower end and fall out into the pan. If the pin comes out, it allows the bearing shells to rotate which cut off a couple of oil holes. IIRC Matt Parkhouse may have written about something like this although I don't see it in my list of articles. I do see an article that Oak wrote in March 2012. Anyway, maybe drop the pan and see if there's something there that doesn't belong?

    Since you're fairly certain there's a bottom end noise, and it's not from the timing chain area, then maybe the only thing to do is take it down so the conrods can be removed for a complete inspection.
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  4. #4
    Registered User bmweuro's Avatar
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    I just did rod bearings and small end bushings as well I have adjusted the custom timing chain tensioner in and out to see if the tone of the sound changes with no luck.
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  5. #5
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    "The noise happens at low to mid RPM under load only while lightly accelerating above 170..........."

    How did you make this observation?
    Were you on the road and thought that was where the noise came from?
    or
    Were you stationary getting the load by braking rear wheel on centerstand or something like that?

    If you were on the road the noise may well be coming from somewhere other than center of engine

    Like maybe universal joint or transmission or horn bashing against something with vibration or tank knocking against a coil

    If you are absolutely sure by stethescope or something like that the knock is in center of engine
    Are you absolutely sure you were not distracted after installing bearing shell? Did you tighten both rod cap screws?

  6. #6
    James.A
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    The last time I heard a noise like that, it was the big end rod bearing on a 1967 BSA Lightning. Same circumstances too. Low to mid RPM under a load.

  7. #7
    Left Coast Rider
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    Quote Originally Posted by bmweuro View Post
    Any ideas on what the noise might be?
    What does your oil look like?

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