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912UL Running Rough

in 912 / 914 Technical Questions 10 years ago

  • Re: 912UL Running Rough

    by » 10 years ago

    How did this vibration start?

    Did you or anyone else do any "servicing" before you noticed it?

    Did it come on slowly over time or was it very noticeable one day?

    How many hours has it been vibrating for?

    How many hours have you flown this engine?

    If you gave it full power from idle, would it hesitate on low revs?

    Cheers Mark


    The vibration built up over 20-30 hours of run time.
    No work other than routine maintenance had been done to the engine.
    Over the last 50 hours I have been chasing the issue.
    I bought the plane with 72 hours on the hobbs. Now near 600.
    No hesitation on throttle input.

    Rebuilding the gearbox helped quite a bit, but didn't return the plane to the smooth operation I had prior.

    Does anyone have any data for what the diameter of the carb slides and the bore should be?
      
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  • Re: 912UL Running Rough

    by » 10 years ago

    Av8r3400

    You said you've rebuilt your carbs a few times, but have you replaced any of the parts or just cleaned and inspected them.

    I didn't realise that Kitfox swapped the manifolds, so as Rob said they will get a rough ride and things will wear quicker than a standard carb set-up.


    I have done both, cleaning, inspecting and minor part replacement (o-rings, etc) on several occasions. The last time complete rebuild kits were installed in both carbs.

    I have a Dynavibe and an electronic carb sync unit that I have nearly worn out processing this motor.
      
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  • Re: 912UL Running Rough

    by » 10 years ago

    Rob-- Can you please tell me what part of the carbs is "ruined" by the vibration from the reversed manifolds? I'm not arguing with you that this is or is not the problem, but before I spend $1500 on new carbs, I want to know what was damaged so that I can confirm this damage and know what to look for.

    In discussing this, if it is just the matter of fact that carbs on a Kitfox IV only last X number of hours before needing replacement, so be it. But, I just want to know what the problem is.

    The pistons move freely in the carbs and are not scored. The bores in the carbs are not damaged. I replaced the diaphragms in the last rebuild. I could not find any play in the throttle plate shaft to body "bushings".
      
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  • Re: 912UL Running Rough

    by » 10 years ago

    the slides get worn: look for chatter marks/metal transfer between carb body and the slide.
    see attached photo.
    Wornslide.jpg (You do not have access to download this file.)
      
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  • Re: 912UL Running Rough

    by » 10 years ago

    Maybe a silly question, but can't you just replace the slide pistons and clean up the bore?

    Does this galling cause the piston to not move smoothly or what? Is this supposed to be lubricated by fuel?
      
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