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912ULS runs very rough above 4200 RPM

in 912 / 914 Technical Questions 10 years ago

  • Re: 912ULS runs very rough above 4200 RPM

    by » 10 years ago

    Thank you Roger,

    No I have not run it up with vacuum gauges, the last differential compression test was about 30 hours ago and it was fine. No it wasn't like this before the carb rebuild :(
      
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  • Re: 912ULS runs very rough above 4200 RPM

    by » 10 years ago

    Hi Robert,

    I would get something is not right in the carbs or carb.

    Since this happened after the carb rebuild then I would certainly revisit this. I would pull both carbs and start looking for what's wrong. It may be the diaphragms are in wrong and the tabs are not lined up, the floats are not at the proper level or the float control arms were put in upside down. I would use carb cleaner in the spray can and flush all passages and follow that up with high pressure air and blow out ALL openings and passages. Make sure the butterfly is in correctly. The center punch mark should be facing out and at the top. Make sure the needle clips are in the #3 position.
    You may need to re-seal the small ballbearing plugs with some epoxy. They may be leaking if you submerged the carbs in a caustic carb cleaner dip or if they are old carbs.

    Do a thourogh job and you'll most likely find the problem. If you just do it half way you may be looking and pulling your hair out for a long time.

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

      
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  • Re: 912ULS runs very rough above 4200 RPM

    by » 10 years ago

    I have had the carbs off several times checking all those things. I will do it again. Perhaps the epoxy I put on the ball bearing plugs is leaking.

    Thanks Roger
      
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  • Re: 912ULS runs very rough above 4200 RPM

    by » 10 years ago

    If you did both carbs at the same time look for swapped choke shafts: the 1-3 carb should have the shaft marker "L" and the 2-4 carb should be marked "R".
    As Roger said look for slide/diaphragm alignment issues. The slides should have the venturi reference holes oriented towards the butterfly.
    Thank you said by: Robert Hirschfield
      
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  • Re: 912ULS runs very rough above 4200 RPM

    by » 10 years ago

    Thank you both Roger and Rob. I have checked all of those things multiple times, but will nevertheless check them again.

    I have a question though. Assuming I find that my work and the work of the mechanic who has looked at my work is done as you both have suggested, is it likely that these symptoms could be caused by a weak coil or some other part of the electrical system other than the plugs (brand new plugs twice now with no actual flying hours on them)?

    Thanks, I will let you know what I find with the carbs.
      
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