Hard to start and idle; Carbs or Petcock?
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  1. #1
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    Hard to start and idle; Carbs or Petcock?

    Hey All,

    My '81 CX500C is being a bear about cold starting; If the bike has sat for longer than about 7 hours, I have to crank it for quite a long time (up to 5 minutes of cranking attempts, less when it is warmer) before it will start again. Once it is running, it has a hard time at idle.

    I have replaced the coils and plugs, did the brass rod mod, replaced the CDI with an Ignitech, and have a brand new battery. I have also replaced the fuel line from the petcock to the carbs.

    None of this has fixed the issue, so I suspect that there is a problem with the fuel getting to the carbs. I have the original carbs and petcock, but both were torn down and got new rubber (and jets for the carbs) 2 years ago.

    Should I be looking to replace the petcock, or do I have to rebuild the carbs again? The petcock will probably need rebuilding anyway as I get a small amount of gas out of it when I change it from On to Off or Reserve, but I was hoping to not have to tear apart the carbs again.

    What do you all think?
    1981 CX500C

  2. #2
    JC
    JC is offline
    Super Moderator JC's Avatar
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    Have you got a vacuum petcock?

  3. #3
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    The OEM one is a vacuum petcock.
    1981 CX500C

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  5. #4
    Senior Member Old Okie's Avatar
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    Get a manual petcock and make sure the choke is working correctly.
    1979 CX500C (Marshall's bike)

    1981 Yamaha XS650 (New Adventure)

    1979 Honda CB400T II (Rescued Bike)

    1972 Suzuki RV90(for the grandkids)Ole Smokey

  6. #5
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    quit guessing and swapping parts

    take the tank off

    hook up a bottle or small test tank

    wait till its cold and see how it starts on the alternate fuel source

    let us know
    GET AN OIL FILTER ADAPTER

    Order The Quick Build Exhaust Pipes


    GET A QUICKSET VALVE TOOL


    Way to many bikes to maintain this list LOL !!

    email me at murray at murrayscarbs.com

  7. #6
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    To be fair, I actually replaced everything on that list except the battery prior to this issue; I thought the sudden issue was the battery (and it was old and needed replacing anyway). The replacements were done to head off any potential weak points in the bike from old parts.

    I will have to wait until tomorrow afternoon to test it again, and I need to find an appropriate container to use for the fuel source in the meantime.
    1981 CX500C

  8. #7
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    After a lot of draining the bowls and trying again, I am pretty sure the issue is that the flow from the tank is the weak point; There is a slight rise in the fuel line from the petcock as it clears over the carb, and the small little spurts that come out of the fuel line take forever to fill up the rise until they finally overflow and actually start the bike.

    I am going to replace the petcock as soon as I can find an acceptable one that isn't vacuum operated. As an aside, if I get rid of the vacuum operated petcock, I presume I need to plug the vacuum hose off the carb to prevent other crap from being sucked in, yes?
    Last edited by Solonys; 03-14-2017 at 06:14 PM.
    1981 CX500C

  9. #8
    Senior Member
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    Use a different fuel source and test it
    GET AN OIL FILTER ADAPTER

    Order The Quick Build Exhaust Pipes


    GET A QUICKSET VALVE TOOL


    Way to many bikes to maintain this list LOL !!

    email me at murray at murrayscarbs.com

  10. #9
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    I actually did; the issue only crops up when the bike is hooked to it's normal petcock. Either the vacuum coming off the carb isn't enough to pull fuel quickly, or the petcock just doesn't want to give up the fuel that easily.
    1981 CX500C

  11. #10
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    good

    as the diaphragm in the petcock gets old it gets more rigid and does not move as well so it does not open as eisily or as fully under low vacuum

    and it gets worse as the rubber gets colder as well
    GET AN OIL FILTER ADAPTER

    Order The Quick Build Exhaust Pipes


    GET A QUICKSET VALVE TOOL


    Way to many bikes to maintain this list LOL !!

    email me at murray at murrayscarbs.com

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