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new carbs venting fuel at 2500rpm and below

in 912 / 914 Technical Questions 12 years ago

  • Re: new carbs venting fuel at 2500rpm and below

    by » 12 years ago

    I just started having the carb fuel vent problem as well. My 912UL mid 90`s #4153.404 with 300 hours total time. Always ran very well. Sync carbs every 40 hours or so. On this forum Roger advised to change up to the "black tipped" float valves which I have just done. The carbs were very clean but when I had them off I sprayed carb cleaner anyway and dried them off with air pressure.
    Today I had difficulty starting because the carbs flooded out the vent tubes when cranking. I have these tubes connected into the air filters. To help the flooding situation and concern about fire, I added a couple of feet of tubing to each carb vent and directed these away from the a/c. It did start and ran very roughly until it cleared out the excess fuel but still spit out the tubes at anything below 3500 RPM or so. As it warmed up, initially it seemed to run more smoothly but after a few minutes it started "loading up" with too much fuel again.
    This engine is pretty much original (including the fuel pump). I read quite a bit about fuel pumps on these forums and some seem to over pressure. I am not sure, but don`t fuel pumps just get weaker as they wear out? Or is it possible my pump may be deciding to just put out too much pressure. I have been thinking of by passing the pump and see what happens. It is a high wing Kitfox and should run on gravity flow ( at least for ground checking) I have not run a pressure gauge on the system.
    I realize this is a long story but would like to narrow down the problem before admitting defeat(don`t like that) and tow the a/c to Vernon.
    Help Roger!!
      
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  • Re: new carbs venting fuel at 2500rpm and below

    by » 12 years ago

    PS
    I also checked to float arms for proper distance as per Roger`s suggestion. The floats float well.
      
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  • Re: new carbs venting fuel at 2500rpm and below

    by » 12 years ago

    Hi Guys,

    There are only a few things that will cause the fuel to puke out the vent tubes. Here the most common and should be ruled out.
    Carbs not balanced and engine running so rough the floats can't control the level.
    Float arm height not correct. 10.5 mm from the edge of the carb.
    Needle valve spring not functioning.
    too high a fuel pressure
    Old, bad or poorly functioning floats.
    Vent tubes located in the wrong place

    I usually go with a carb balance and a float level check first thing. If you have one of the new fuel pumps then fuel pressure needs to be checked.

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

      
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  • Re: new carbs venting fuel at 2500rpm and below

    by » 12 years ago

    Hi Roger
    Float arms were checked. They were at 10.5MM and required no adjustment. There is no question that the engine runs rough. I did a sync at 3000 rpm but shortly after the carbs started spitting fuel again and of course got much worse for roughness. It was very easy to see the fuel spitting with my 2 foot extensions away from the engine heat.The fuel starts spitting even when the engine is just cranking over before it starts.
    It sure seems like excess pressure to me. What do you think about bypassing the fuel pump (ground run only,of course)
    Cheers
    Don
      
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  • Re: new carbs venting fuel at 2500rpm and below

    by » 12 years ago

    PS. I suspect the floats are original, but they float with out a problem. This engine has run very well,very smooth on idle or cruise, has not been subjected to dirty fuel. No ethanol or AV gas, always unleaded 87, filtered at the hand pump as it is going in to the tank.
    Do not understand how things would change that quickly.
      
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