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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 Mike,

    Thanks for your info. What is interesting about my problem is that it only started about 12 months ago. Before that the engine always ran smoothly. It is very intermittent in nature and can go of its own accord.

    In that case I would focus on the engine mount. Something has changed about how the engine vibrations are damped. I would be concerned that the problem - whatever it is - is progressing so that the vibrations are now present in other phases of flight.

    If you do figure this out, let us know. I'm always interested in learning why I was wrong. :) No doubt the problem will turn out to be a waterlogged prop blade or something random like that.
      
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  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    The vibration problem has been getting slightly worse over the last few flights but new information has come to light.

    Today I noticed that with the engine off but the electric fuel pump on, fuel could be seen gently seeping out of the float bowl gasket of the port carb. Removing the air filter and looking into the venturi revealed fuel coming up past the needle.

    My initial thoughts are that the float needle is not shutting off the fuel completely when the float bowl has reached the correct level. This in turn causes the bowl to become pressurised forcing fuel up the main jet turning my carburated engine into a fuel injected one. This will make the mixture rich on one half of the engine. This probably explains my vibration.

    The float needle and seat look OK as far as I can tell. Does anyone have any experience of how best to proceed from here?

    I will probably print out the relevant pages from the Rotax manual and check the fuel level in the float bowl, etc.

    Kevin
      
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  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    Hey Kevin,

    Do you have a fuel pressure gauge on the airplane? Check to see what the fuel pressure is when it happens. Sometimes as you throttle back the fuel pressure will increase for a moment. Couple that with a warn float needle valve, you may be getting transient high pressure fuel getting through the valve causing a momentary rich mixture. As you know, these engines can sometimes vibrate pretty well creating warn float chamber parts. Just a thought....now, back to football.

    Russ
      
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  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    Hi Russ,

    Yes there is a pressure gauge but the reading never varies at any phase of the flight.

    I have had the carbs on the bench today and fed them with fuel which I have pressurised. The one that leaked on the plane still leaked but the other one was fine.

    I decided to swap over the float needles and lever arms. This cured the problem on the port carb. I then changed the lever arm back and it was still fine. I then refitted the original float needle and this time no leak! However, looking in the float bowl there was some black debris. This may have been flushed out by my experimentation. I blow through the needle valve with compressed air and rebuilt both carbs. Both now appear to be leak free.

    You can hear the fuel filling the bowl initially and then it goes quiet when the floats shut off the valve. There is no longer any sign of fuel leaking from the bowl or coming up through the main jet.

    I am reluctant to drive the 2 hour round trip to the plane only to find I have the same problem again but I cannot think what else to do. All of the parts that I inspected looked OK so I am in no hurry to start replacing items for no good reason.

    Kevin
      
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  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    Hi Kevin,

    This may or may not be the cause of your original vibration issue, but in addition to.

    If you know you have a fuel by-pass at the needle then I would ask how many years and hours do these carbs have on them and have they been rebuild any time recently? If they have several years on them or high hours then it may be time for a rebuild.
    It could also be something as simple as a float level adjustment or as you described a simple cleaning.

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

      
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