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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,

    You may or may not notice much of a change between a 155 and a 158. It would depend on your instrumentation and altitude you fly out of and your average cruise altitude. You should have the 158 in a 912UL and the 155 in a 912ULS. The Bing 64 does a pretty fair job verses a standard carb in adjusting to altitude, but before I would suggest someone make a jet change they would really need to know at what altitude they will normally fly out of (field elevation) and their average cruise altitude. If you fly out of Los Angeles, CA at sea level that's one thing, but if you fly out of (live their) Leadville, CO at 9927'MLS then that's a whole new ball game. Then you need to have the proper instruments to watch what's going on in your engine. A CHT and or Water temp isn't even close.
    I like digital instruments for many things over analog because they read much faster and you may see a problem start to manifest before it does damage. For the huge majority of 912's in the world and the lack of true understanding on jet functions and lack of good instrumentation using the Rotax factory jets and settings is usually a good idea. If you're really up on your game then tweaking can help under extreme flying conditions.

    Rotax4.pdf (You do not have access to download this file.)

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

    Thank you said by: mike hallam
      
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  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    Still no luck with the vibration problem. I installed a new throttle cable today, commmercial one from Aircraft Spruce designed for the Rotax 912. Balanced the carbs, went for a fly. Have to say it ran smoother than the old cable setup but still got the vibration with closing the throttle. Occured some minutes into the flight, not straight away after climb out. Would go with working throttle, mainly closing it completely then opening again. Would sometimes occur with increasing the throttle from established cruise. All temps and pressures fine. CHT both around 80 deg C. No EGTs. Have recently pulled all plugs. All were fine appearing except front right which looked a little rich/cold. It is the coldest cylinder in theory. All gaps checked and fine. On descent to land, engine ran well throttle full closed. Idles well on ground too. Ran up to 5000rpm on ground and can't reproduce the problem at all on the ground. Back up pump on/off makes no difference in flight. Both vent tubes to carbs are routed as per rotax manual.

    So, in summary, it mainly occurs with reducing throttle, in flight only, usually some minutes after climbout after have established cruise. Occurs worst if open throttle straight/level then close it. Goes away by closing throttle more then re-opening.

    Things still to try/check :

    Left carb bowl not looked in yet.
    Trial of removing drip trays (think problem started after installing drip trays), not sure how this could make a difference.
    Check tightness of engine mount bolts (imagine this would still be a problem on the ground too though)

    If these things don't help, I may have to look at disassembly of carbs next.

    Any bright ideas guys?

    Paul
      
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  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    Food for thought,

    Sometimes we think it absolutely has to be one thing and then we find out it was another.

    could this be airframe or prop or gearbox shimming?

    Just something to rule out.

    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

    Alright. Shot in the dark. A sticky piston (slide, plunger) in one of the carbs. I've seen this once, black goo impeding the motion of the piston. As you close the throttle the piston should move down in the bore but if one piston sticks you'll get different fuel flow on each side of the engine.

    Not too difficult to check. No need to remove the carb from the engine.
      
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  • Re: 912UL Engine vibration on throttle back

    by » 12 years ago

    tell me more about gearbox shimming? this is a brand new motor with 80 hrs on it now, would something go wrong with the gearbox so soon? only run on shell sport 4 plus. prop is wooden, 2 blade, fixed pitch.conceivable it could be the problem, would it now occur on the ground too though? i did consider the only difference in flight is that the prop transiently drives the motor with its inertia on throttle closing but the vibration i get is happening also (albeit less often) with throttle opening. strange that it doesn't occur until minutes into the flight. how does one check the carb piston slide? does the engine need to be running?

    cheers

    paul
      
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