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Rotax 912 ul, severe vibration

in 912 / 914 Technical Questions 8 years ago

  • Re: Rotax 912 ul, severe vibration

    by » 8 years ago

    Idle and main jets and their respective housing inspected and clean.

    Float valves are closing, pressure test performed as per manual.

    I can't recall the needle position from the top of my head but I do remember we looked at it. As per the manual, checked on both carbs. (3rd from the top?).

    I would be really surprised (and disappointed with my mechanic) If finally this is carburetor related because it's been inspected many many times, trying new components, different carburetor sets, etc.

    To add even more confusion (at least to me) this is how my spark looks like. I doesn't look right, seems like is missing: www.youtube.com/watch?v=0s61h6TgHYE

    The ignition check has always been correct. The original ignition system on the 912 ul was inspected and checked by replacing all its components. Finally it was totally replaced with the one of a 912 uls without any improvement, same vibration.

    Eventually we discarded that this was the origin of the vibration but rather another consequence of the rough running.

    What do you think?
      
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  • Re: Rotax 912 ul, severe vibration

    by » 8 years ago

    I have seen weak and missing sparks with a test similar to yours in the video. In my case it turned out to be faulty ignition leads. I replaced them and that problem was solved.

    In your video the spark does not look weak so I doubt that you have a problem with the ignition leads. If the sparks are missing occasionally then your problems could be: wiring, ignition module, coil, inductive pick-up, ignition supply generator (red wire).

    Given that you have replaced most parts of the ignition system it is unlikely to be the coils and the modules. Did you replace the inductive pick-ups or check the gap to the flywheel?

    If you have access to an oscilloscope you can check the pulses coming from the inductive pick-ups and then the output pulse to the coil primary. The pick-up pulses need to be a minimum of about 10V when the engine is running normally. When cranking only 2V is required. I don't know the amplitude of the coil primary signal or the voltage from the generator.

    It might be worth looking at all of your spark plugs to see if they are all 'missing' like the one in the video.

    Good luck!
      
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  • Re: Rotax 912 ul, severe vibration

    by » 8 years ago

    Hi Kevin,

    the pick-ups were checked with a multi-meter at first and installed making sure they where tighten in the appropriate sequence and that the gap was correct. Then replaced by the 912 uls set. (gap as per the manual).

    I had issues with the cables you mentioned from the stator to the CDI (ignition supply). There is a SB from rotax regarding the isolation quality of these cables and mine were indeed gummy, with some open spots. I got the cables fixed, since I couldn't find a spare with the old style connections, and later on replaced the full stator assembly with the 912 uls one and the vibration kept the same

    Believe me when I say I was euphoric when I found this since I though I have finally fixed my plane! but the exact same vibration persisted, no change on the amplitude, noise, shake, etc ...

    .
    Unfortunately I don't have access to an oscilloscope. The spark looks more or less the same on the four cylinders and this is why I though the missing would be normal on low rpm's during start up.
      
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  • Re: Rotax 912 ul, severe vibration

    by » 8 years ago

    Jorge, did you install the 860990 outer cover on the ignition wires?
    If you don't use this you will get arcing and misfire.
      
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  • Re: Rotax 912 ul, severe vibration

    by » 8 years ago

    Good grief Jorge, it looks like you really have tried just about everything! Whilst your video shows missing sparks at cranking speeds it does not show whether you have missing sparks at normal engine running speed. I guess that a random misfire or multi-cylinder misfire would produce rough running symptoms.

    Could an in-line spark tester be used to see if each plug is firing? Something like this:

    https://www.amazon.com/Lisle-20610-Inline-Spark-Tester/dp/B0002STSC6

    Is there anything that you haven't swapped out?

    Kevin
      
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