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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

    Kevin,

    Could be an air leak. Most obvious place would be the carb boots. You might also check the carb diaphragms and the O rings on the butterfly shaft. Air leaks will be more obvious at lower RPMs when the manifold vacuum is highest. One way to check for air leaks is to spray a little starting fluid around the boots and O rings and see if the engine speeds up. I would sync at the lower RPMs and not worry about slight differences at WOT. Paul may be right also. it could be engine mount rubbers. What is your idle speed? Sometimes if you get much below 1800RPM, you will get some vibration. Have the carbs been cleaned lately? if not, you could have differences in fuel air ratios between carbs. This would show up pretty quickly in EGT readings. Worn jets or jet needles could also cause air/fuel imbalances.

    This may not be a single problem. could be several small things together causing the problem.

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

    by » 11 years ago

    Nothing seems to have helped therefore another hypothesis.

    May be there is nothing wrong!

    Due to the throttling back the engine is less pulling or even windmilling and could result in vibration problems. I have the same behaviour when on downwind I reduce the throttle (FK9ELAs having relative soft engine mounts). Also play of the prop and its blades could result in vibration. When I pull the stick (engine is pulling more) when vibration occurs it immediately stops fueling my hypothesis. I have accepted this behaviour as it does not present a safety problem.

    I have read also in this thread that Kevin has cleaned the carbs using pressurised air. When the carbs are not fully dismantled it could have dislodged the membranes.

    Jan
      
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  • Re: 912UL Engine vibration on throttle back

    by » 11 years ago

    Hey Jan I am starting to think you are right. I have tried many things including new prop, numerous carb balancing, replacing carb rubber mounts, engine mounts etc. I still get the vibe when slowing down/closing throttle but it will also stop if I raise the nose making the engine pull rather than the prop 'windmilling'. I notice it only happens if I have been flying relatively fast in cruise despite my machine not being a particulary fast machine! I have flown in another 912 powered machine which displayed the same characteristic but with much sturdier engine mount design than mine and it was much less noticable. The Skyranger mount system is pretty 'soft' in that you can move the prop hub in all directions quite a bit compared to other 912 installations I have seen. I will just do my best to avoid it happening by gradual throttle closure keeping the nose up when starting the descent. I am interested to hear if Kevin ever came to a conclusion regarding this 'characteristic' of the rotax. Is it because of the reduction gearbox? I never noticed such a thing with the last direct drive aircraft I flew.

    Paul
      
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  • Re: 912UL Engine vibration on throttle back

    by » 11 years ago

    Paul,

    I do not think it is related to the gearbox. Just relative soft mountings and aerodynamic excitation due to the prop: a resonance phenomenon.

    Jan
      
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  • Re: 912UL Engine vibration on throttle back

    by » 11 years ago

    Gents,

    Here is an update of what I have done so far:

    1. Rebalancing the carbs, multiple times.
    2. The gauges have been checked for accuracy against each other.
    3. The carbs have been stripped and examined multiple times.
    4. The needles, jets and diaphragms have all been replaced.
    5. The carb mounting rubbers have inspected multiple times.
    6. The gearbox was reshimmed about 200 hours ago.
    7. The float bowl fuel level has been checked multiple times.
    8. Spark plugs look good, fault occurs on either ignition circuit.
    9. The prop does not look like it is any different from before the fault.
    10. Prop blade angles checked.
    11. Throttle cables checked for smooth operation.
    12. Throttle angles checked for balance at all throttle settings.
    13. Engine mounts appear in good condition.
    14. I experimented with the part load needle in different positions with little or no effect.
    15. Piston spring length checked.


    What is most frustrating is that the engine ran very smoothly for the first 9 years of its life.
    At the moment I am just flying the plane and tolerating the vibration :-(

    Unless it gets worse or any new information comes to light I will probably just put up with the problem for now.

    Kevin
      
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