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Petcock, Vacuum, Carb?

Discussion in 'The Garage' started by Addison, Nov 25, 2019.

  1. Addison

    Addison XS400 Enthusiast

    Well I took the boy into the garage tonight to take a look at the old bike.

    We started by attempting to install a new inline fuel filter.

    Yes, we installed it correctly - however - when we turned the petcock to the 'ON' position, gas would not flow down the line, thru the filter, and into the carb.

    But, when I disconnected the fuel line from the carb, then turned the petcock on, the gas would flow.

    In all cases, the bike was not running. Does this mean that the vacuum feature in the petcock was functioning correctly? I started the bike, and it fired up, but then eventually died out as expected.... no gas supply.

    I disassembled the petcock and all is fine there.

    Am I missing something really simple here? Any help at all would be appreciated.
  2. xschris

    xschris a lifestyle not a trend Top Contributor

    You hooked up the vacuum line to the intake boot and started the motor. That's how these work. There is a prime setting with them that lets fuel flow with the motor off. Make sure the gas tank has enough gas for the petcock work on the on setting otherwise you would have to use the reserve setting. It's best not to let the tank get that low as any junk in the tank could plug up things.
  3. Addison

    Addison XS400 Enthusiast

    Hi XSChris thanks for the reply.
    The gas tank was approximately half full at this point.
    I started the bike, with the petcock open, (so assuming a working vac system at this point, the carb should start sucking fuel down into it) but the gas did not flow thru the line.
    Is it possible there is some debris or something clogging the carb fuel intake line? It was working fine before I took the gas line off and then installed the fuel filter.
    Kind of stumped on this one.
  4. Addison

    Addison XS400 Enthusiast

    I just realized, that perhaps, the carb already had enough fuel in it and therefor wasn't 'calling' for fuel at that point... perhaps if I let the bike run longer, then the vacuum would have worked properly and then would have started sucking fuel?
  5. Gra900

    Gra900 XS400 Addict

    That’s not really how it works. The vacuum is created by the pistons going up and down, the vacuum then pulls against a diaphragm in the petcock that opens the valve in the petcock and let’s the fuel flow. The vacuum isn’t needed to suck fuel into the carbs, that just goes down by gravity when the float drops down a fraction and opens the float valve.
    xschris likes this.
  6. Addison

    Addison XS400 Enthusiast

    so, there is something wrong with the fact that gravity isn't pulling fuel from the petcock to the carb then. I understand it will reach a point (or should reach a point) where it won't continue to flow, but at this point, the gas won't exit the petcock until i disconnect the fuel line from the carb, with the petcock open.
    Is it possible to upload a video to this site? I could easily take a vid and show you guys what i'm talking about.
  7. XSLeo

    XSLeo XS400 Junkie

    With the fuel line disconnected, turn petcock to on, fuel flows correct? With a non vacuum petcock that sounds right.
    With a vacuum operated petcock that should not happen. On your petcock what positions do you have?
    Vacuum should have a PRI, ON, RES. A non vacuum petcock should have Off, On, RES.
    If gas flows out of the line with it unhooked it should flow while hooked. try again. with line hooked up. unscrew one of the carb float bowl drains. Fuel should run out.
    When you installed the fuel filter di you just add it to the line that was there or did you add more line.? it's best to have the fuel line be as straight as it can be. It should not droop below the carbs.
    When you disturbed the fuel line you may have knocked loose some crud. This crud may have moved down to the carbs. Plugging the float valves.
  8. Addison

    Addison XS400 Enthusiast

    Thank you Leo! Will give that all a shot. Appreciate you walking me thru that.

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