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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 » 11 years ago

    Hours aren't always a deciding factor. You may need a gearbox re-shim or you may need new engine mounts. How many years old is this?

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

      
    Report

  • Re: 912UL Engine vibration on throttle back

    by » 11 years ago

    I spent some time over the weekend looking at my rough running problem. I have discovered that with the vacuum gauges attached the pressures to not track evenly over the rev range. At the higher rpm the vacuums are the same, in the mid range there is about a 1 inHg difference.

    Is that sufficient to cause a problem and if so what can cause it?

    I did make some measurements of left & right throttle angle with different throttle settings. They seemed to track evenly over the entire rev range so I don't think that there is anything funny going on with the cables or springs.

    Kevin
      
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  • Re: 912UL Engine vibration on throttle back

    by » 11 years ago

    Kevin,

    At full throttle, vacuum readings are almost nonexistent but as you reduce throttle, they become more pronounced. Reason for this is that at full throttle, if one throttle plate is open 100% and the other is open 98% It is only a 2% difference. At idle however, if one throttle plate is open 2% and the other is open 4%, that gives you a much higher vacuum in one carb even though the percentage difference between plates is still only 2%. That is why it is much easier and more important to sync the carbs at lower rpms. Get the pneumatic sync right at 2500 or 3500 RPM and you will be on the money at full throttle.

    Bill.
      
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  • Re: 912UL Engine vibration on throttle back

    by » 11 years ago

    Bill,

    If I sync at lower RPM the vacuum between the sides is different at higher RPM. If I get it balanced at higher RPM then it is out at lower settings.

    I can only think that this could be caused by an air leak somewhere past the throttle butterfly.

    Kevin
      
    Report

  • Re: 912UL Engine vibration on throttle back

    by » 11 years ago

    Hi Kevin,

    You shouldn't have any issues syncing at idle and 3300-3500 rpm and don't worry about every thing else. If you can't sync the carbs then you either have some debris in the carbs and they should be removed and thoroughly cleaned or you have a leak. Debris is more common. If the carbs are old then they should be rebuilt and if they have just been worked on I would take them apart and find out what isn't right. Your clips in the needles should be in the #3 position.
    You should also check and make sure your gauges are equal. Your gauges may be wrong.

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

      
    Report
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