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petcock help

Discussion in 'The Garage' started by wrench234, Oct 4, 2012.

  1. wrench234

    wrench234 XS400 Enthusiast

    i am wondering about the petcock on my bike it has pri,on,and res. on and res are
    obvious but what is pri never had anything that had that thanks in advance for helping
    with my remedial question sorry:confused:
    also i was wondering what the purpose of the vacuum line to the petcock was for i see that there is a diaphram in it from some
    searching i have done and i need to rebuild mine
  2. Lou Ranger

    Lou Ranger Former xs400 Luddite Top Contributor

    OK, I get to give this a try and then I will be graded by the folks who know for sure. Wait for their answers before you take the info out of your RAM and retain it.
    I think ON means that fuel flows from the tank into the carbs. But ON is only on when the engine is on and the vacuum goes up that little line and sucks some sort of diaphragm valve open (ON).
    RES is for when you run out of gas because the pipe feeding the vacuum operated valve is at a higher level in the tank than the fuel level. Moving the petcock to RES accesses a lower opening which allows fuel to flow by gravity into the carbs so you can go directly to a fuelling station.
    I think PRI stands for prime which is where you turn it if you are totally out of fuel and need to fill the carbs up before moving the petcock back to "ON". Why one wouldn't just use RES to accomplish this is beyond me.
    Hope I passed. :D
  3. nico

    nico 78 XS400-2E

    Not 100% but I think reserve is the same as on(vacuum) just lower feeding filter inside tank. The prime is free flow as stated. Again, not completely sure but I believe this is the way the original Petcocks were designed.
  4. wrench234

    wrench234 XS400 Enthusiast

    i think i understand, with it in the on or reserve position there needs to be vacuum from the engine to pull diaghpram to allow fuel flow and then prime is straight thru no vacuum needed. sounds complicated really why not just on,off,reserve lol
  5. Lou Ranger

    Lou Ranger Former xs400 Luddite Top Contributor

    Sounds right. I would give myself 2 out of 3 and half marks for the other 3rd = 83%.
    I think that if you had it on gravity feed with no vacuum valve to shut it off then you would be relying on something in the carbs to keep the gas from just filling the bowls and overflowing into somewhere. Lawnmowers seem to have gravity feed tanks with no vacuum actuation. I assume the carb designs needed to make a bike engine go to 10k are configured differently from lawnmower carbs. I'd love to know how though.
  6. wrench234

    wrench234 XS400 Enthusiast

    on the dirt bikes and stuff ive had they just had on,off,res the needle would be pushed up into the seat by the float as fuel filled the bowl then shut flow off
  7. HoughMade

    HoughMade XS400 Guru Top Contributor

    If the original petcock is working, you just leave it on "On". That's the reason for the vacuum petcock- just leave it alone unless there is a reason to do otherwise. That being said, mine went bad and I replaced it with a good old "On", "Off", "Reserve" manual petcock.
  8. wrench234

    wrench234 XS400 Enthusiast

    mine needs rebuilt it leaks i plan on fixing the original one i just was wondering how it worked never had anything before with vacuum controled petcock thanks for the help
  9. kebabs

    kebabs XS400 Enthusiast

    just bought one of these bikes as a project and your absolutely right. making something difficult that dosnt need to be.

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