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vacuum actuated petcock

Discussion in 'The Garage' started by spills, Dec 2, 2017.

  1. spills

    spills XS400 Member

    Hey y'all.

    This week my petcock started pouring gas out the side, where the plastic spacer meets the square housing.

    Upon taking it apart, I found theres a premade hole in the diaphragm, as well as an outlet port- a half round channel about 1mm, heading diagonally down and out of the petcock.

    What is this port for and why did it just start happening? is this an overflow for when the diaphragm is too clogged?
     

    Attached Files:

    Last edited: Dec 2, 2017
  2. kshansen

    kshansen XS400 Addict

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    Question about this petcock. What year bike is this off? As the one I have off my 1977 that I am starting to parts-out looks noticeably different. One point is on mine the fuel outlet nipple on the bottom is cast right in a cover with four JIS screws holding it on. See picture:
    fuel outlet.jpg
     
  3. spills

    spills XS400 Member

    1979 XS400(F?)

    So after reinstallation, petcock seems to be functioning normally.
    Still stumped as to why that slot is there; what its function is.
     

    Attached Files:

  4. kshansen

    kshansen XS400 Addict

    237
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    did you see my "conversation" reply about the parts?
     
  5. BBS360

    BBS360 XS400 Guru

    His looks the same as the one on my '82, rather than my xs360 that's the same as yours.

    It's been a while since I've had the petcock apart but they work based on the difference in atmospheric pressure on one side of the diaphragm vs lower "vacuum" pressure on the other side.
    Without that atmospheric reference it wouldn't work reliably.
     
  6. xschris

    xschris miles of smiles =)

    Yep, that's why it's there. My 79-82 has it also. I think they started being made like that on the 78-E model. Make sure you put it back together the way it was. The orientation of that hole make a difference.
     
  7. spills

    spills XS400 Member

    I put it back together the way it came, and it first seemed to be fine, but then after a few turns started leaking again.

    This to me means that I need to rebuild with a new diaphragm, so I'm looking for a rebuild kit.
    Any other advice here?
     
  8. BBS360

    BBS360 XS400 Guru

    You can get a new non-vacuum petcock for about the same price as a rebuild kit.
     
  9. xschris

    xschris miles of smiles =)

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