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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

    :whistle:


    One thing I like about this forum is that the discussion here is very civil. So now you follow up one comment which was frankly uninformed with another which is insulting.
      
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  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    Hi guys

    The problem with the vibration didn't resurface after I stretched the springs back to their original length. That does not necessarily mean that this was the cause of the intermittent vibration, as it happened only once in every say 10 throttle-backs, so it might be that I was just lucky to have missed it in subsequent flights. The new carburettor kit might indeed have been the final fix.

    If the diaphragm spring is shorter than original I would ASSume that the steel's molecular structure was altered sometimes during the hundred thousand cycles it went through during the long life of my engine and I wouldn't expect a spring "repaired" in such a manner to have the same spring coefficient as a new one. I was willing to take my chances, but how far you take "experimental" to mean just that is obviously up to each pilot.

    Blue skies
    Arthur
      
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  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    If the carb slide spring became elongated or shortened during its life it would be better to just buy a new one. I'm not sure that lengthening or shortening will give you the same results and it may easily cause it to be out of whack with the other carb spring.

    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

    I am still having the same vibration issue but now can be reproduced on the ground too. I invested in a new prop and it is incredibly smooth compared with the old 2 blade wood. New prop is ground adj 3 blade "Kool". It is pitched to 5100 in climb, max 5600-5700 SL. It is staggering how much smoother it is at 4500 and above. However, I still get vibration between 4400-3800 on throttle back, and throttle forward it recurs and then like the snap of your fingers comes smooth/clean again at about 4300. If going slower I can get the engine to run smooth in the 4000-4400 range but a sudden gust of wind on the nose will start it off again. On finals, the engine runs rough anywhere between 2300-3500 like is missing a beat. A bugger when you just want a little extra blip of power to clear the fence. Still idles beatifully. I have noticed that the right carb has fuel venting on landing via the tube. The left side doesn't seem to do this. I don't have a fuel pressure gauge or a return line. Motor is now 160hrs since new a year ago. I removed the carbs and inspected the jets, cleaned with carb cleaner and compressed air. All looked fine and after refitting the problem is still there. Rechecked the carb sync and all fine and remains balanced all the way through the vibe zone on the ground, between 2200-3000rpm. I think the smoother 3 blade prop has now made me aware that the vibe is reproducible on the ground (albeit in diff rpm range), either that or the problem is getting worse. Idle needles set at 1.5 turns, tried richer and leaner but no effect. Also tried needle clip in leaner position with no effect other than made engine backfire on shutdown. O rings between carb rubber mount and intake manifold are new. Just wondering if I have high fuel pressure causing some flooding of right carb, would that account for the problems? Also, will try new engine mount rubbers in case they are too soft as there appears more motor movement on my installation with turning prop then other planes I have looked at.
      
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  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    If the rubber engine mounts are over 5-6 years change them. If you have a new style fuel pump you need the re-circulation line. Either the engine is running rough and making that carb vent or the carb is flooding and making the engine rough. I think I would focus on that carb first. When you rebuilt the carbs did you replace the Viton tip needle valve?

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

      
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