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extreme vibration at 4800 rpm

in 912 / 914 Technical Questions 13 years ago
My 914 vibrates alarmingly at about 4800 when throttle is opened and load is on. Does not want to increase revs (thank goodness). Runs perfectly at lower revs and got me home !
Motor 108hrs.
New plugs, dual ignition OK, both fuel pumps OK, fuel pressure OK..NOT the prop.
I suspect a carburetor problem.
Any clues please ?

  • Re: extreme vibration at 4800 rpm

    by » 13 years ago

    Hi Richard,

    This may or may not be the carbs, but if you want to check them then an easy quick check is put the gauge on and check the carb sync. Run it on up higher than normal and see if you have a big discrepancy in vacuum. This will give you some idea if it's a fuel flow issue and or a carb issue. This will give you at least a starting point for a diagnosis. Depending on what you find it may lead you in the correct direction or rule it out.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

      
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  • Re: extreme vibration at 4800 rpm

    by » 13 years ago

    Lee, please explain how this can differentiate between a carb and a fuel flow issue.
    Thanks for your reply. I need my hand held.
    Cheers
    Richard
      
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  • Re: extreme vibration at 4800 rpm

    by » 13 years ago

    Hi Richard,

    It's the difference in vacuum on the gauges. You can even tell if it just involves the idle circuit. I have tried the electronic gauges, but you can't diagnose with them, tell how much of a change you want to make on a cable or which carb you want to adjust just looking at the electronic devise. The gauges tell me all of that, not mention which carb is having an issue.
    The electronic devices work, but they don't tell you anything except when it is in sync.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

    Thank you said by: Richard Cooper
      
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  • Re: extreme vibration at 4800 rpm

    by » 13 years ago

    Any oil dripping from the airbox drains? If so you have a turbo oil scavange or check valve problem and it has contaminated the carbs.
    If that is OK does it bog down/backfire in the 4800rpm range? If so that is caused by too lean a mixture (go one notch richer on the needles)
    Rob
      
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  • Re: extreme vibration at 4800 rpm

    by » 13 years ago

    Good morning Rob. Thankyou for your interest.
    No oil from the airbox drains.
    It bogs down shaking badly in 4800 range. Does not backfire.
    I examined the spark plugs and one bank does appear a little blacker. I then examined the carb diaphragm on the blacker side but everything looked perfect.....bowl spotless etc.
    I will get a gauge and follow advice given by Roger Lee and do a vacuum test.
    Cheers
    Richard
      
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