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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

    The carbs are about 9 years old and have about 700 hours on them. they look in excellent condition and at every 200 hour strip down everything has been as it should. They have run without issue until this latest vibration issues started.

    I don't have the gauge specified in the manual for checking the float level but they measure 10.5mm in the manner that others have indicated on this forum.

    Kevin
      
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  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    Hi Kevin,

    With 9 years on them you may have the old red tip needle valve and with 9 years and 700 hrs. I would rebuild them. A lot of years are as harmful as a lot of hours.

    Have you already gone through and checked the other suggestions from some of the other post?

    The most common vibration causes are carbs out of sync, gearbox needs re-shimming, old engine mounts, prop out of balance (or blades not equally pitched).

    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

    Hi Roger,

    The needles have black Viton tops and they look perfect. There is no sign of wear on them and the material 'feels' a sensible hardness.

    I have checked all of the other suggestions (thanks folks). The carbs have been synced regularly (as well as when this problem occurred), the gearbox was re-shimmed a couple of years ago, the engine mounts are in good order, the prop blade was checked last week. The only one that I have not done is to dynamically balance the prop. That said there has been no loss or gain of material on the prop since before the problem began and it is not wood.

    I think that now that I no longer appear to have the leak on the starboard carb I will try them on the plane again and if I have further grief I will invest in a couple of service kits.

    Kevin
      
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  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    Hi,

    I spent some time investigating the vibration problem today and with the vacuum gauges connected I could not get the carbs to stay synced at all RPM. I also noticed that throttling back from 4000+ RPM down to less than 2500 RPM the engine becomes rough and then smooths out. One of the vacuum gauges shows some oscillation when the engine is rough.

    Does anyone have any additional ideas? I am reluctant to embark down the path of changing parts until the fault goes. I would rather diagnose it properly.

    Kevin
      
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  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    Hi Kevin,

    You may have nothing wrong. It is just a change in air/fuel flow and down at a very low rpm. The 4000 you used is more representative of flight and 2500 of idle.

    Even with the gauges on and you in sync the engine will still be slightly rougher than when the balance tube is connected. There can be a slight rough spot in this rpm range and is why I like to sync at 3500+. The fuel air flow is too low when down at the 2500 and less to give a good sync for the higher rpm set. You just noticed what I have always said. If you sync at 2500 rpm then when you advance to the higher rpms where we fly and where we have a better representation of a fuel air flow the carbs won't be in a good sync. We don't fly at 2500 rpm and usually idle at warm up (2000-2400) closer to that so why sync that low with a minimal amount of fuel/air flow.
    Sync them at idle and then around 3500. Re-connect the balance tube and the rest will take care of itself.

    If you try to make the gauges stay together from idle to 4000 you may not get there and each engine may be slightly different. Like a difference in personality. Doesn't mean anything is wrong and throwing parts at it may not get you a spec closer.

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

      
    Report
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