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Why is there air in my fuel line?

Discussion in 'Hangout Lounge' started by tabaka45, Sep 17, 2019.

  1. tabaka45

    tabaka45 Well-Known Member

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    I have clear fuel line with an inline filter on both of my bikes and there is an air bubble in each line. One is in the filter and I can sort of understand a little air getting caught in the filter since it is a little flat on its side. however, the one on my XJ is right where the line comes out of the petcock. Even on prime the bubble will not go away. Fuel is passing and both bikes run well, but I am curious about where the air comes from and why it doesn't float up through the fuel line and petcock and into the tank.
     
  2. Polock

    Polock Well-Known Member

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    is the fuel line empty, mine goes empty after a week or so. then i use prime to fill it and it starts, with the bubble in the filter. that's the last i think about the bubble.
    i'm not sure if it's there when i get back or not, i'll have to watch that
     
  3. tabaka45

    tabaka45 Well-Known Member

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    No, the line never empties. The bubble just stays in the same place and fuel goes by it. I do make sure that the line always has an upward slant to it so a bubble won’t get trapped and cause an air block. It just seems to me that it would clear the line and float up into the tank.
     
  4. jayrodoh

    jayrodoh YimYam Premium Member

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    All my bikes with a clear line have a bubble at the highest point. Most have a petcock that exits horizontally, here's the way I've always understood it. When you first hook the line on, it is empty and full of air. Air will rise but due to the narrowing of the diameter of the nipple inside, the air has no where to go. As you've mentioned, bike will run fine and if you disconnect the fuel line from the carb and put the petcock on prime, you will see fuel flowing like a river under the air bubble. I think the air comes from three places, first being that the line is empty to begin with, second if the carbs empty enough while sitting to open the float valves air would rise up, and lastly there may be some air off-gassing from the fuel itself.

    Crappy paint drawing:

    Bubble.png
     
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  5. tabaka45

    tabaka45 Well-Known Member

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    I tend to agree with you, but there's a lot more air in my line than in your picture.
     
  6. jayrodoh

    jayrodoh YimYam Premium Member

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    My XJ has been off the road for almost two years so I can't look but if I recall correctly I had quite a bit of air too. Could be something in the design of the pet cock, maybe the feed hole behind the valve is lower than the output of the petcock. My KZ has a large air bubble, maybe 3/4 of the hose diameter, in the line where it goes up and back down to carbs after the filter. I wouldn't be concerned about it, gas will always flow under/around it as needed.
     
  7. tabaka45

    tabaka45 Well-Known Member

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    I'm more curious than concerned. I have been running with a bubble every since I got the bike. Just seems odd to me to be designed that way.
     
  8. Polock

    Polock Well-Known Member

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    well then it's time to get out the b.s.
    that's the super deluxe filter with altitude compensation, as the altitude changes the size of the bubble changes to automatically adjust fuel flow.
    :)
     
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  9. Dave in Ireland

    Dave in Ireland Well-Known Member

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    People with black rubber hoses never worry about air bubbles.
     
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