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Petcock / filter ?

Discussion in 'The Garage' started by RobAch, Jul 29, 2020.

  1. RobAch

    RobAch XS400 Member

    Hi All,

    Pulled off the petcock while attempting to clean the rust out of the gas tank. See photo. In the pri position I can blow air thru 84CD1025-A64F-4189-AA3C-5D44EC9D3EC3.jpeg the hoses back thru the petcock but not in the on or res position. Is this correct? Also, any idea where I might be able to obtain a new in tank fuel filter? Would it be easier to just place two inline in the hoses to the carbs? Any direction would be greatly appreciated.
  2. amc49

    amc49 XS400 Enthusiast

    I see what looks like ethanol damage to plastic.

    Yes you can use filters in the fuel line downlegs.

    The petcock is supposed to flow in both positions. Just not in off. The res position will be the low one and likely plugged with tank rust.
  3. xschris

    xschris a lifestyle not a trend Top Contributor

    It's a vacuum operated petcock. It needs a vacuum in order for the on and res to flow. It comes from the motor when it's running and hooks to the air barb on the back then to the intake manifold. I would just use a in-line filter instead of the in-tank ones as they are easier to replace. Those in tank ones fall apart after 40 years of use.
  4. RobAch

    RobAch XS400 Member

    Thank you both. Greatly appreciated. Any recommendations on inline filters? There’s only about 5 inches of hose to the first carb.
  5. xschris

    xschris a lifestyle not a trend Top Contributor

    This is what I use. I get them at my local yamaha dealer.
  6. amc49

    amc49 XS400 Enthusiast

    'It's a vacuum operated petcock.'

    I thought of that but the general shape picced there did not lend toward it. A shot from a different angle would have showed the shape likely. I wondered because 'prime' is generally an override for a vacuum piece involved in the system. Maybe it's a remote.

    Vacuum setups cause so much trouble on the bigger Hondas we typically defeat them to go back to old school but you MUST train yourself to ALWAYS turn the petcock off at parking the bike or there can be trouble.

    Maybe it's the filter plastic type used, mine are in fine shape after 40 years but ethanol can fix that in a couple years, I sold lots of plastic fuel strainers for cars when in parts that were only 3-4 years old, they simply crumbled to the touch. The ethanol acids do it when the fuel is allowed to phase separate.
  7. RobAch

    RobAch XS400 Member

    So if placing 2 new inline filters or vacuum issues deters fuel from free flowing to the carbs, technically I would be able to leave the petcock in the prime position as long as I turn it back to either on or reserve, stopping free flow, when the bike is off, assuming of course I get this running?

    I think I’d actually prefer that type of off/on valve.
    Last edited: Jul 31, 2020
  8. xschris

    xschris a lifestyle not a trend Top Contributor

    The stock petcock has one fuel line and one vacuum line. The lower one goes to the carbs and the other thinner one on the back goes to the barb for the right side intake manifold. When the motor creates vacuum it allows fuel to flow from the on and res settings. PRI is for filling carbs when the motor is off or your vacuum line is broken. https://www.ebay.com/itm/Yamaha-Petcock-Fuel-Gas-Tap-Valve-Right-or-Left-XS650-1978-84-SR-500-2FastMoto/201951225444?ssPageName=STRK:MEBIDX:IT&_trksid=p2060353.m1438.l2649 These are what I use on most of my sohc xs400 bikes.

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