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Carb ventilation tube is leaking fuel

in 912 / 914 Technical Questions 11 years ago
Hi,

There is a tube to connect the carb bowel with the airbox. My understanding is that this is to ensure there is same airpressure in the carb bowel compared with the intake airpressure.

However this tube is leaking little fuel which is now thrown into the airbox again. This clearly isnt good causing too rich mixture and potential rough running (especially in lower RPM's)

Does anybody have any experience with this (leaking fuel out of that tube) and what is causing this?

  • Re: Carb ventilation tube is leaking fuel

    by » 11 years ago

    Hi Eelco,
    Your engine may be running too rough for the floats and needle valve to control the fuel level in the bowl and it will puke fuel through the vent. You may need to do a carb sync or you may need to replace the Viton tipped needle valve. These vent tubes do need to get equal air pressure to both carbs and should never be stuck into the air stream. Do the cheapest easy thing first, do the carb sync and make sure both vent tubes are getting the same pressure from their location, i.e. the air box.

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

      
    Report

  • Re: Carb ventilation tube is leaking fuel

    by » 11 years ago

    Eelco,

    If it is something that happens on rare occasions, it could be just the result of taxiing on a rough field or a rough idle and is nothing to worry about. If it is happening constantly, you might check your float valve for trash or a damaged viton tip. Also check the floats for bouyancy and proper float arm adjustment. If the fuel pressure is too high it can override the floats and float valve, causing the same problem.

    Bill.
      
    Report

  • Re: Carb ventilation tube is leaking fuel

    by » 11 years ago

    Thanks Bill and thanks Roger,

    Some good feedback - appreciated

    Main problem is rough running in low RPM. We are not sure yet about the cause but it seems to be caused by the vent tubes. Mainly in low RPM. During cruise flight and take-off no issues. But it does happen some time during approach/landing as well and that are moments when you actually do not want a rough running engine

    Carbs have been replaced with other carbs (just as a test) and that did run without any issues. So most likely indeed something with the floats etc.

    We will take your advice on board and get things further checked
      
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  • Re: Carb ventilation tube is leaking fuel

    by » 11 years ago

    I don't know how much time you have on your carbs (years or hours), but you may need someone to take them apart and check them. Float level can be your problem or something in a passage way. Instead of spending tons of hours doing all this work, just pull them and clean and inspect them. You should be done then. If you keep going trying to diagnose or trouble shoot you may end up with 10 times the work and time invested.

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

      
    Report
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