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912UL Engine vibration on throttle back

in 912 / 914 Technical Questions 12 years ago

  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    Back again. A change in the pattern of the vibration issue. Flew for 1.5 hrs this morning. On returning home, had to climb to 3500 ft to clear some hills. Conditions mild, calm. Climbed under 5000 rpm power, levelled off at 3500, 4700rpm and all ok. About 5 minutes later, vibration started (just as I was crossing the hills, Muprhys law). Came on with no change in throttle setting. May have started just after hit a few mild bumps. Showed no signs of clearing itself so I opened throttle to 5000rpm and it went. When closed throttle again to 4700rpm it returned so I continued to close throttle and descended on other side of hills to 1500ft, opened throttle again to same rpm and no further recurrence all the way home. Again, all temps and pressures normal. Bizarre it is.

    Paul
    Thank you said by: Richard DeHaven
      
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  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    Kevin,

    You might investigate Mike's theory about the engine mounts. I've seen problems on other engines where vibrations occured at varying times and speeds and were cured with new isolation mounts.

    Bill.
      
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  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    Good point Bill. Engine mounts get hard and loose their dampening qualities. This is the reason to change them out at 5 years.

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

      
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  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    With my own carb problem, I have now fitted the service kit to the carbs, so I have effectively replaced all the seals, diaphragms, needles and jets. I am hoping to go an try them out tomorrow.


    One more thought. You have an older 912, have you done the carb upgrade per SI-912-012? This includes the addition of a vibration damping o-ring to the jet needle. This o-ring stops the jet needle vibrating around and is missing in older 912 carbs.
      
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  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    The engine mount rubbers are only 6 months old so won't try changing them yet. I removed the left carb drip tray and noted the O ring between the drip tray and intake manifold was broken, therefore an intake air leak. My drip trays were not supply with any gaskets so I made my own and reassembled with a thin smear of permatex. The engine ran the sweetest at idle for a long time. Thinking I had fixed the problem I went flying. After climbing to 2500ft and throttling back the vibration returned. This time at a higher rpm setting though, 5000rpm. Engine ran sweet otherwise. Will look at right carb drip tray today to see if any air ingress there too. Still plugging away.....

    Paul
      
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