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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

    Hi Kevin

    Why do you need to open the throttle fully when doing this?

    Mark
      
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  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    Mark, you are correct.

    There is no need to open the throttle as the butterfly valve is on the opposite side to the air filter.

    I was thinking back to when I test the pistons on a pair of carbs that I had off the plane but that still had the air filters attached.

    Kevin
      
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  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    I had the same problem with a 912ULS engine in a Van's Aircraft RV-12 E-LSA after only 95 hours of Hobbs time. I traced the problem to severely deformed viton rubber tips on the float bowl needle valves in both carburetors. The deformation was severe enough to allow the needle valves to penetrate into the needle valve almost 1/16 inch which allowed the fuel level to rise to the point of flooding the engine.

    I had to replace both needle valves at about $38 each. It solved the problem instantly. I now have about 25 hours of flight time on the new needle valves and am anxiously awaiting to see if I'm going to have to replace needle valves every 95 hours!!!!!!!
      
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  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    Update:

    Yesterday I ground ran the engine again with the newly overhauled carbs.

    The roughness is still there between 2500 & 3000rpm. Pulling the choke either independently (for each carb) or together does not help this roughness.

    Removing the spark plugs, the insulator was still white indicating a lean mixture.

    I was unable to fly due to a water logged runway so I cannot determine whether my original rough running problem when throttling back from cruise power is still present.

    I might be able to fly at the weekend if the runway has recovered sufficiently.

    Kevin
      
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  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    Well, I am gonna just stop hunting for the cause of the vibration and just keep flying until it becomes more clear what the cause is (i.e. if it gets worse/more persistent!). I flew this morning in cool stil air and no vibration. Flew again when hot and bumpy and had one episode when closing throttle slightly as hit surge of air causing plane to gain altitude. Didn't last long as closed then re-opened throttle and no further recurrence. Engine running well otherwise. I tried removing the nose wheel spat a few days ago as this had been annoying me somewhat working itself loose overtime. It is pretty close to the prop and gets a good splattering of bugs on one side so thought it may have been a source of vibration. It seemed to make a difference until today when another episode happened. It could be the prop hitting some resonance but don't have the luxury of trying another prop just now so will box on and fly as normal. Have not tried removing the drip trays as just can't see it making a difference. Again, even on a warm day today the Temps of oil and CHT were in the 80's deg C.

    Any progress at your end Kevin?

    Paul
      
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