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Overflowing carbs on a 912UL

in 912 / 914 Technical Questions 6 years ago
Hi all. What are the primary reasons that a carburettor would overflow? Is it possible for fuel to overflow out of anywhere else other than the vent tubes assuming that the float bowl gaskets are in good condition? Is there any reason that both carbs would leak fuel whilst trying to start a cold engine on a very cold day (excessive vibration etc) but then stop leaking once the engine is started? Thoughts would be appreciated.

  • Re: Overflowing carbs on a 912UL

    by » 6 years ago

    Hi Shelley,
    Bad vibration can cause carbs to vent fuel because it doesn't allow the floats to control the fuel well.
    Other things that will cause fuel venting is sinking floats (allows too high a fuel level in the bowls) and float armature not set correctly (high fuel levels) and of course bowl gaskets needing replacement. This one is certainly more common.

    Your engine should start with a turn of the prop and should not be cranking over and over. Once started it should not be shaking. It may be a tad rough until you take the choke off and increase the rpm to at least 2K+ rpm, but should be smooth after that. If you have soft start it will feel a little rough for 5-6 seconds then be smooth.

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

      
    Report

  • Re: Overflowing carbs on a 912UL

    by » 6 years ago

    Thanks Roger. I was trying to start a 912ul trike on a cold day. The engine seemed to be turning over more than fast enough but made no attempt to fire. After many attempts I noticed fuel leaking from around the mixture screw area (not from the vent tube) from both carbs. Tried flooding procedure but still would not fire. Checked float brackets thinking fuel was not shutting off and they were actually sitting too low and shutting off valve too early. Ended up jump starting and fired up first time. I can't work out why fuel would leak out on a failed start from both carbs on choke?
      
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  • Re: Overflowing carbs on a 912UL

    by » 6 years ago

    I also checked the plugs expecting them to be soaked with fuel but they weren't. Is it possible for fuel to push out of the idle mixture screw,
    or must it have come from somewhere else?
      
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  • Re: Overflowing carbs on a 912UL

    by » 6 years ago

    If you were just cranking and the engine shaking it may just have been the carb bowl overflowing since the floats were bouncing around and having a hard time controlling the level. It could also mean you need to check the float weights and or the float armature that helps the floats control the level.

    No matter what it does sound like you need a new battery or put a good charge on the one you have.

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

      
    Report

  • Re: Overflowing carbs on a 912UL

    by » 6 years ago

    Thanks Roger.
      
    Report
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