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Hello, I am a new member of your site and want to have your advice about the following question:

I bought a 2006 FK9 with Rotax 912 ULS(100Ps) and I just saw your video about Carburetor Vent Lines and found mine going into the 2 airfilters. Is this dangerous and should I change this immediately by buying new filters and fix the tubes like in your video?

According to an accident with the same configuration the NTSB was writing this: "

Examination of the engine revealed that the carburetor float chamber vent lines had been incorrectly routed to the air filter"

Best regards

Thierry

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  • Re: Carburetor Vent Lines NOT into Filter

    by » 4 months ago


    I have the same K&N filters on my Eurostar and the veent pipes are routed externally as shown in your second picture. I don't know whether you should change your installation to match. I'm sure that others will be able to enlighten you.


    Thank you said by: THIERRY WEIS

  • Re: Carburetor Vent Lines NOT into Filter

    by » 4 months ago


    The standard configuration is to have them just tucked under the bail, as in your second picture; you just want them to have exposure to ambient air pressure.  I would take them out of the filters; I can't think of any good reason for them to be routed there. 

    You may be able to patch the hole in your filter to avoid buying new ones, although I do not think new ones are very expensive.


    Thank you said by: THIERRY WEIS

  • Re: Carburetor Vent Lines NOT into Filter

    by » 4 months ago


    It depends on your airplane configuration.  The vent lines must be at the exact same external air pressure as the float bowl pressure, otherwise the carbs will be too lean or too rich. Most carbs with K&N filters are set up with the vent line under the bail wire of the float bowl. Best recommendation is discuss this with a Rotax mechanic.  Also check out eaa.org/Videos, Dean Vogel (from Lockwood) "Rotax 912 Fuels and Carburetion". His videos explain in detail the mysteries of the Bing carb and Rotax engines:)


    Thank you said by: THIERRY WEIS

  • Re: Carburetor Vent Lines NOT into Filter

    by » 4 months ago


    I would say the pressure at the vent lines is what determines the float bowl pressure...  and you want that to be ambient, not higher or lower.


  • Re: Carburetor Vent Lines NOT into Filter

    by » 4 months ago


    The vent lines exist to equalize the pressure between the carb bowl and the Carb external throat.
    This allows for the venturi effect in the carb throat to draw the proper amount of fuel from the bowl.
    The Carb is calibrated assuming a zero pressure loss across the air filter.
    As the filter ages, the pressure drop across the filter medium causes more fuel to be drawn from the float bowl causing an increasingly richer mixture over time.
    If the vent tube is attached to communicate directly wih the pressure internal to the Air filter, as the filter ages, the Bowl pressure changes equally to compensate for the change and the mixture remains stable over time.

    The engines with Airboxes installed between the Air Filter and the carbs, have their vent tube connected to the air boxes and not to the ambient air.
    This arrrangement is especially important if the engines have turbochargers installed.

    There is absolutely Nothing wrong with connecting the vent tube to the air filters.
    This is actually the preferred configuration.

    What is unusual is that the Bing Carbs were designed with external vent tubes and the bowls were not vented to the carb external throat via a simple drilled hole in the carb body connecting the bowl to the throat like most other carbs have.


    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Bill.Hertzel@Yahoo.com
    Clicking the "Thank You" is Always Appreciated.


    Thank you said by: THIERRY WEIS

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