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My last 912 ULS powered aircraft had the carburettor float bowl breather tubes venting into the engine compartment - over 10 years, never a problem and never noticed any fuel exiting from the tubes.

My new aircraft/new engine has a carburetor air box. The breather tubes are connected to the air box. The air box has a drain point, on each side, at the bottom of the air box. The drain points each have a tube, connected by a "T" piece, into one tube which terminates in a "catch can". The catch can will accumulate about 20ml fuel per one hour flight.

This is a relatively recent occurrence, with no fuel being collected during 5 hrs of pre test flight ground runs.

I have weighed the floats and found them to be just within specifications.

The engine is running well.

I am inclined to think that the floats have had time to absorb some fuel and are now less buoyant than when new. This is effecting how quickly the floats are closing off the float valve and possibly not providing the earlier pressure to close off the valve.

My questions:

Can fuel exiting the float bowl ever be considered normal?

Is the "fix" - removing the carburettors from engine, inverting carb, removing float bowl, measuring the float suspension brackets - adjusting if necessary?

  • Re: Carb Float Bowl Breather/Overflow

    by » 3 weeks ago


    Float weight "just within spec"....  Does that mean < 7 grams per pair?  Are you using an accurate scale?  I would suspect heavy float buoyancy problem.  Float bowl on aircraft engine is a very violent place to maintain precise fuel level.  I bought several sets of Bing floats and finally purchase Marvel Schebler epoxy floats and have had no problems since.

    The guys riding BMW motorcycles with Bing carbs close the fuel tank valve when parked in garage for fear of fuel overflow.  Several instances of fuel all over the garage floor...

     


  • Re: Carb Float Bowl Breather/Overflow

    by » 3 weeks ago


    Fuel bubbling through the vent line is a good indicator that the floats are too heavy and the float chamber is flooding. Carefully remove the float bowl and check the level, it shouldn’t be full.You don’t have to rotate or remove the carb to replace them.


  • Re: Carb Float Bowl Breather/Overflow

    by » 3 weeks ago


    The floats can be heavy, but it could also mean the brass float armature is out of adjustment. The carb needs to be upside down to check the proper height with a millimeter ruler. 10.5 mm from carb edge to the top of the brass armature. Carbs way out of sink or something causing the engine to shake can cause the vents to puke fuel too.


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


    Thank you said by: Sean Griffin

  • Re: Carb Float Bowl Breather/Overflow

    by » 3 weeks ago


    I would stress - This Is a Brand New Engine. Purchased 2019, first started in mid 2022 now has about 5 hrs ground running, including pneumatic carb balance, and now has a total of 17 hrs ). The fuel in catch can only started once flight testing began.

    Engine appears to be running well, without vibration, delivering full power, EGTs are even - there is a bit more exhaust soot than I expected.

    Just Within Specification's - means the float are 6.83 gms & 6.91 gms,  just below the 7 gms max allowable weight.

    Scales - purchased jewelers digital scales, specially for this test - I can attest to consistency but not to accuracy.

    In my installation, removing the float bowl, without spilling fuel, is near impossible - could mean that fuel level is a bit high, equally the difficulty in keeping bowl level, as it is removed. Aircraft is a tail wheel - may be contributing factor. In short hard, to assess fuel level.


  • Re: Carb Float Bowl Breather/Overflow

    by » 3 weeks ago


    MARVEL-SCHEBLER MS80-430 FLOAT KIT FOR BING CARBURETOR

     

    https://www.aircraftspruce.com/catalog/eppages/tempest_ms80-430.php?clickkey=3050658

     


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