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Fuel spitting out of air filters at full power

in 912 / 914 Technical Questions 12 years ago

  • Re: Fuel spitting out of air filters at full power

    by » 12 years ago

    Hi Neil,

    What is the WOT rpm when you are flat and level?
    That will help determine a good prop pitch setting. Having un-synced carbs causes more vibration which will not allow the carbs to control the fuel well because of the shaking floats, so the excess fuel will go spill out the vent tubes and or carb. Think of the 912 as two engines. A right side and a left side engine. If one carb is out of sync it may be trying to control that side at 5000 rpm while the other side is trying to control it at 5100+ rpm. The numbers are examples only, but you get the picture. One side is having a battle with the other which causes vibration and over time can cause damage and excessive wear. The cross over tube or balance tube can help correct some of this, but it is not a cure all and these need to be balanced. Once you do it a time or two it's fairly easy, especially on your Gyro because of the engine setup and the way it faces.

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

      
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  • Re: Fuel spitting out of air filters at full power

    by » 12 years ago

    Hi Roger,

    Not sure what the WOT rpm is. Never had a need to push that hard when level. I'll be out tomorrow so i'll open her up and revert.

    I have heard that these engines are sensitive to vibration. There is not a trace of fuel coming out of the overflow pipes or anywhere else, but i now can see how the imbalance might cause the problem. I have a carb balancer, but it's still in the box. I'll get it out at the earliest and plumb it to the engine pronto. As you point out, access is easy so i have no excuses to delay.

    Thanks again.

    Neil
      
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  • Re: Fuel spitting out of air filters at full power

    by » 12 years ago

    WOT means wide open throttle

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

      
    Report

  • Re: Fuel spitting out of air filters at full power

    by » 8 years ago

    Hello to everybody.
    I would like to re-open this thread as my 912 UL is showing the same problem and I am not able to find the cause for such a behaviour.
    My engine has run now for 1400 hours without problems. From about 100 hours (two years) I find a yellow/brown stain on the engine cowl and on the fuel collection trays in the air filters area.
    I found that both carburetors did not pass the float needle pressure test so I suspected flooding of the carburetors as cause of the stains.
    I installed two brand new carburetors but that did not solve the problem.
    I perfomed the differential compression test getting 78-79 psi over 80 psi for all four cylinders. This should exclude leaking intake valves.
    I have an inflight variable pitch prop and fly at cruise in WOT condition at 5150-5200 rpm.
    At take off I get about 5500-5600 rpm.
    My next test will be changing fuel type ( from regular Mogas to 100 octane Mogas).
    Any idea to help me?
    Is it possible that although the differential compression test is ok, I could have some leakage or valve overlap at cruise now that the engine is getiing old?
    Thanks.
    Sebastiano from Italy
      
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  • Re: Fuel spitting out of air filters at full power

    by » 8 years ago

    Hi Zeb,

    Just an FYI. I wouldn't use the variable pitch prop to only get 5100-5200 WOT and fly at WOT. This is over pitching the prop and depending on the crankcase you have (mid 2006 and earlier) it could crack it. This over pitch will also cause higher engine temps, extra fuel use and stress other parts. I would set that prop to get at least 5500-5650 rpm at WOT then throttle back to 5200.

    Are the carbs in sync so they aren't vibrating a lot?
    The fuel in the the drip tray can come from a couple of sources. The float level being to high and the fuel pukes out the vent tube. I would pull the carbs and turn them upside down and check the float height or could be a sinking float. Bad carb bowl gaskets. A leak from one of the small ball bearing sealed ports that should have some epoxy on them. This one isn't very high on my suspicion list. The vent tubes not positioned correctly. Your leak isn't fuel type caused so I would stick with 91 Oct.

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

      
    Report
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