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1980 GS550 Petcock Operation Question

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    1980 GS550 Petcock Operation Question

    Hello all,

    New to the forum. Recently bought a 1980 GS550; trying to get it ready for the spring. While trying to sort out starting issues I have directed my attention to the petcock since I found gas had enter the crankcase (already changed the oil and filter). It has the dreaded '80 no-lever petcock.

    I have been searching the forum but so far all I see is "get a new unit". I am looking for some help understanding the operation of this "thing" to make sure it is bad before I buy a replacement.

    What I know:
    - It only has "On" (horizontal) and "PRIme" (vertical).
    - Prime needs some cranks/vacuum pulses to start the fuel flow.
    - The "On" position is vacuum operated, so fuel should not flow if the engine is off.
    - There is no reserve.

    When I turn the position to "On" (horizontal), fuel is still coming out. While testing it out of the gas tank I noticed that it will hold a vacuum if the hole in the front is covered (see sample pic below). That sounds to me like the diaphragm is still in good condition, but I am not sure if that helps in any way to understand why fuel still comes out while set to "On". If the opening is not covered air just goes in through there.

    '80 petcock.jpg

    Looking for any ideas on what may be happening with this unit.

    Thanks,

    #2
    If fuel is flowing without vacuum then the O-ring on the diaphragm has failed. Another thing to be aware of is that the back cover has a check valve under a pressed in cover. If/when the check valve fails you won't have a functional Prime function anymore.
    Ed

    To measure is to know.

    Mikuni O-ring Kits For Sale...https://www.thegsresources.com/_foru...ts#post1703182

    Top Newbie Mistakes thread...http://www.thegsresources.com/_forum...d.php?t=171846

    Carb rebuild tutorial...https://gsarchive.bwringer.com/mtsac...d_Tutorial.pdf

    KZ750E Rebuild Thread...http://www.thegsresources.com/_forum...0-Resurrection

    Comment


      #3
      Good morning and welcome to the form,
      This petcock mainly consist of the diaphragm, a spring, and an o-ring on the portion of the diaphragm that stops the fuel flow. You can probably remove the four screws and clean it up and check that o-ring for proper sealing and the spring for proper tension. The diaphragm might also be stiff restricting movement.
      1978 GS 1000 (pods, V&H 4 to 1, Dyna S, Dyna coils, stage 3 jet kit, Progressive springs, relay mod, 530 chain, Honda regulator, clutch basket welded and shimmed)
      1970 Honda C70

      Comment


        #4
        Turning the slot to the prime position closes off an air bleed, located through the center of that slot. As long as the check valve holds, the vacuum from cranking is supposed to hold the diaphragm and fuel valve open. In the run position, the air bleed is open to the diaphragm and requires a constant vacuum signal (like a running engine) to hold it, and the fuel valve open.
        Mine has never really worked right in prime, if the bike’s sat for too long, I have to pull the vacuum line off the carb and take a pull of it.... at least you know real quick if the diaphragm is in tact or not.
        -1980 GS1100 LT
        -1975 Honda cb750K
        -1972 Honda cl175
        - Currently presiding over a 1970 T500

        Comment


          #5
          Thanks for the replies. I will post some pics of the inside today when I get home form work. Maybe I'll use some Harbor Freight o-rings I have to see if that helps. It if does, I'll get the Viton ones.

          Comment


            #6
            Originally posted by Gumpster View Post
            Good morning and welcome to the form,
            This petcock mainly consist of the diaphragm, a spring, and an o-ring on the portion of the diaphragm that stops the fuel flow. You can probably remove the four screws and clean it up and check that o-ring for proper sealing and the spring for proper tension. The diaphragm might also be stiff restricting movement.
            So.... I had found a spring while cleaning the workbench on Sunday and decided to save it. Turns out it was the spring needed for the petcock diaphragm to stay closed. Once installed, the o-ring pressed against the top and I was able to close that path. However, while testing the two positions I still noticed I needed to put my finger over the front opening to hold a vacuum. Playing around with a flashlight and a small nail, I found out the check valve was long gone. There were a few pieces of black goo in there, but no valve. Getting a new petcock; case closed. Thanks for the help.

            Pic for reference.

            20180206_212638.jpg

            Comment


              #7
              That was the best fix you can do, is buy a new one.
              Last edited by mrbill5491; 02-09-2018, 01:21 PM.
              sigpicMrBill Been a GSR member on and off since April 2002
              1980 GS 750E Bought new in Feb of 1980
              2015 CAN AM RTS


              Stuff I've done to my bike 1100E front end with new Sonic springs, 1100E swing arm conversion with new Progressive shocks installed, 530 sprockets/chain conversion, new SS brake lines, new brake pads. New SS fasteners through out. Rebuilt carbs, new EBC clutch springs and horn installed. New paint. Motor runs strong.

              Comment

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