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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

    Hi Roger

    I agree 100% that the sprag clutch is damaged through kick back.

    The problem is the overload clutch causes the kick back unless the set up is perfect and the friction torque is set as high as possible.

    You also need to fit the Rotax soft start modules or the conair soft start module to try and prevent kick back.

    If you don't have a slipper clutch fitted, you don't get sprag related starting issues.

    A standard 912 is better off with out the slipper clutch.

    I am not saying that every engine that has an overload clutch fitted will have sprag issues, but I will say you won't get sprag damaging kick back if the engine does not have an overload clutch fitted.

    Next time you get an engine that won't start because the sprag has failed and it is almost impossible to start because of kick back, before you do anything, change the gearbox set up, just as an experiment, and see what happens.:-)

    Mark
      
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  • Re: 912UL Engine vibration on throttle back

    by » 11 years ago

    We need some clarification of the term kickback. To me it is the violent stopping of the prop/engine when the sprag slips. This is the classic sign that your sprag has is worn and cannot carry the crank through the compression/ignition phase.
    If this is true then the kickback is a result of the worn sprag, not the cause.
    To me the true cause is:
    1-Engine mount design that allows the engine and carbs to shake at start-up causing poor atomization of the fuel and raw fuel in the intake manifold. (airbox should have been mandatory for the 100hp 912S/ULS as it helps to stabilize the carbs).
    2-Poor starting technique.
    3-Incorrect idle setting.
    4-Loose gearbox.
    If your term kickback refers to any botched start then I agree it is the cause of sprag wear.
    Rob
      
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  • Re: 912UL Engine vibration on throttle back

    by » 11 years ago

    Kevin,

    Its up to you mate, your call as to fit or not fit the clutch.

    Good luck
      
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  • Re: 912UL Engine vibration on throttle back

    by » 11 years ago

    I agree with what Rob said about kickback and its causes.
    It stretches out the sprag spring and then fails to engage or stay engaged properly.

    Common causes as Rob stated:
    1-Engine mount design that allows the engine and carbs to shake at start-up causing poor atomization of the fuel and raw fuel in the intake manifold. (airbox should have been mandatory for the 100hp 912S/ULS as it helps to stabilize the carbs).
    2-Poor starting technique.
    3-Incorrect idle setting.
    4-Loose gearbox.

    I'll add two more for sprag wear:
    5- Bad or weak starter (like the old black starter wasn't as up to the task as well as the newer higher torque stater that has now been around many years.
    6- Weak battery causing the pilot to use excessive cranking.

    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 » 11 years ago

    Hi Kevin, bummer to hear about your gearbox. I still have that vibe with throttle back, when it seems that the prop starts driving the motor vs motor driving prop. I have 200 hrs and it is not getting worse. I would like to hear if your gearbox repair fixes the issue and if it does I will get mine looked at too. I will be pretty peeved if after less than 200hrs since new my gearbox had worn in anyway though!

    Paul
      
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