vented gas cap leks fuel?

Discussion in 'Motorized Bicycle Welding, Fabrication and Paintin' started by Big Bill, Sep 6, 2012.

  1. Big Bill

    Big Bill New Member

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    Need help

    I have just finished building a tank out of .062 sheet metal and I
    installed a Bung King vented gas cap. The problem is it leaks fuel out
    of the breather hole when I am riding, some times when I put gas in it and install the cap it will shoot fuel out of the vent hole when I put the cap back on.I will not fill it all the way up ( about an 1 inch from the top). Today
    I only had a half a tank and it was leaking out of the vent hole while riding
    it home. The only thing I can think of is modify the cap so I can install a
    vent tude and run it under the bike.

    thanks Bill
     
  2. shwnrttr

    shwnrttr New Member

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    are you missing the filter like this? 4d296876.jpg
     
  3. Big Bill

    Big Bill New Member

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    Re: vented gas cap leaks fuel?

    Yep thats the one, and it has a screan at the bottom just like the picture.
    Boy this thing likes to leak on me. lol!!
     
    #3 Big Bill, Sep 6, 2012
    Last edited: Sep 7, 2012
  4. GearNut

    GearNut Active Member

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    All I can suggest is using your idea to run a tube off of the cap itself or installing an auxiliary vent in the fuel tank. An internal tube running from the top of the tank near the filler hole and out the bottom floor of the tank. Like a stand pipe would be installed.
    This is how modern *sold in California, street legal* motorcycle tanks are vented. It is the only way the Mfg's can vent the tank through a vapor canister to comply with the CARB reg's. The "vented" caps sold in California only vent air in, nothing out, so as to not cause a vacuum in the tank as the fuel level drops.

    Yeah, it sounds like alot of extra work, but if you wanna keep the look of the cap clean without a hose on it...

    I have a suspicion that you are developing pressure inside the tank as the gas sloshes and vibrates inside, causing it to vaporize some. The pressure has nowhere else to go but through the gas soaked screen in the cap forcing the gas out of the screen and out the vent hole. The stand pipe vent would solve this problem.
     
    #4 GearNut, Sep 6, 2012
    Last edited: Sep 6, 2012
  5. sportscarpat

    sportscarpat Bonneville Bomber the Salt Flat record breaker

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  6. culvercityclassic

    culvercityclassic Well-Known Member

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    I have the same setup as Pat, I also install the breather tube with a check valve in it. I purchase them from the local motorcycle shop.

    The vent in the bung king work poorly, we have all dealt
    with its issues.
     
  7. Goat Herder

    Goat Herder Gutter Rider

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    So the tubes work better than constructing some kind of anti slosh baffle? Or do you two, Sports and culver incorporate both into the design set up?
     
  8. sportscarpat

    sportscarpat Bonneville Bomber the Salt Flat record breaker

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    A large anti-slosh baffle may work just fine, but I have not tried them yet. The small internal check valves, ones inside the breather of the cap, help but do not correct the problem completely. The external barb fitting and hose is really simple and works. I also find different engines have different vibration zones that lead to problems. The big four strokes, at least for me, seem to have venting issues low in the rpm range while high winding two strokes leak more at high speed cruise. I find the hose fix works well in both situations.
     
  9. Big Bill

    Big Bill New Member

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    Thanks for the help, I an going to add a breather tube to the
    cap. I will have the final build pictures for you this week end.

    Thanks again Bill
     

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