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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 » 11 years ago

    Just a note.
    Engine mounts can look perfect, but we have no way to check the duro which is their MFG'ed hardness. It still may be a good idea to replace these. It may take only 1-2 to cause your problem. If they are at least 5 years old I would replace them regardless of hours. Sometime when we rule out everything then what ever is left no matter how improbable is usually the problem.

    It's just food for thought as you try to work through your problem.

    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 » 11 years ago

    Have you checked the diaphragm spring length? :evil: I think that is where my problem was...
      
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  • Re: 912UL Engine vibration on throttle back

    by » 11 years ago

    the springs are set by force not length. small differences in length are expected.
    Rob
      
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  • Re: 912UL Engine vibration on throttle back

    by » 11 years ago

    Hi Kevin

    Just a thought, I know you have had your gearbox serviced 200 hrs ago, but it is possible that the plastic thrust washer has failed and so your gear box preload tension will be way out. It would be worth removing the gearbox to check this out. It could be removed and replaced in less than an hour.

    Mark
      
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  • Re: 912UL Engine vibration on throttle back

    by » 11 years ago

    Hi Kevin

    I have the same problem and mine started around a month ago. On a full power 5500rpm climb it runs sweet as a nut and always has. One day as I throttled back past 4800rpm I felt a vibration that felt like some form of resonance. Increasing the throttle up through 4800rpm to full power did not cause the vibration.
    I thought it might be the carbs, so I have changed all the jets, diaphragms, springs, o rings, float valves, gaskets etc.
    It still vibrated.
    I balanced the prop to within a feather weight.
    It still vibrated.
    I have swapped both CDI units for 2 new ones.
    It still vibrated.
    I fitted a new set of spark plugs.
    It still vibrated.
    I fitted 8 new spark plug caps.
    It still vibrated.
    I had a crack in the exhaust, so I fitted 2 new exhausts.
    It still vibrated.
    I have fitted new engine mounts.
    It still vibrated.

    The vibration can be replicated on the ground by revving the engine to around 4500 rpm and then slightly reducing the power by 50rpm, or by doing a mag drop. Once it starts to resonate, I need a big increase or reduction in power to stop it.

    I tried a different prop and there was no vibration. This prop was set so maximum RPM is 5000rpm. This is what the aircraft manufactures recommend.

    I increased my prop by 3 degrees to the same setting and the problem has gone away.

    My problem is why did the vibration start in the first place. One day it was perfectly OK, then it just started to vibrate.

    The engine has 2100hrs, the prop is a Warp Drive 3 blade pusher.

    Any ideas.

    Mark
      
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