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912UL Running Rough

in 912 / 914 Technical Questions 10 years ago

  • Re: 912UL Running Rough

    by » 10 years ago

    Hi Av8r3400/Rob

    Last year I had a resonant vibration at around 4500 rpm when in flight, and around 3800 when static on the ground. After looking at all the obvious possible causes I removed the gearbox again and decided that the wear on the splines of both the prop shaft and corresponding overload clutch bore were probably was excessive and that they needed changing.
    After renewing nearly everything in the gearbox the vibration stopped.

    Believe me I went through everything and replaced or temporarily swapped most things.

    The vibration only happened on throttle reduction.

    I always keep on top of the gearbox torque as this is a prime cause of sprag clutch spring damage.

    Mark
      
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  • 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
      
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  • Re: 912UL Running Rough

    by » 10 years ago

    Rob

    I am amazed that that amount of wear on the splines and splined bore did not cause any vibration.

    It may have run fine but i'm sure if there was some more testing you may get the sort of resonant vibration I was getting.

    It would only happen when the power was reduced slightly. It could be made to happen by reducing power, doing a mag drop, reducing airspeed within a certain power setting. It was a problem as it was happening at around cruise power.

    I didn't imagine it and the gearbox was the issue.

    How many hours were on the parts shown in the pics?

    Mark
      
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  • Re: 912UL Running Rough

    by » 10 years ago

    Mark,
    Only 1297 TSN. It was abused. it failed 4 fuel pumps over several months. the cause of the pump failures was conclusively the pitting on the gearset: this pitting was shooting a harmonic frequency that would "float" and break the reed valves. The cause of the pitting was found to be poor maintenance and operation: 100hr oil changes with $2.00 Qt oil from WallyWorld and excessive low idle combined with low friction torque.

    AV8r: I am not suggesting spending any more good money on your current carbs. The Kitfox 4 has the intake manifolds swapped to the outboard position so the carb center of mass is also outboard and they get trashed. I hate to say it but they are probably junk and need to be replaced.
      
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  • Re: 912UL Running Rough

    by » 10 years ago

    Rob

    I know you get what you pay for with things like oil, but that "takes the biscuit".

    Friction torque is so important and I think this proves it. Scary.

    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.
      
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