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Thread: Screw inside carb valve

  1. #1
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    Screw inside carb valve

    I have a 1980 cx500c. Inside of the top of the carb is a valve with a needle coming out of the bottom. When I pulled the carb off the needle would move freely up and down. I looked inside the top of the valve and saw a screw that wasn't screwed in so I screwed it in. Now the needle is secured and not moving. I'm curious, which way is it supposed to be? What is the purpose of the needle?

  2. #2
    Super Moderator ramprat06's Avatar
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    Your description is hard to follow, exactly. Do you mean the float bowl needle valve? Take and post a good picture of what/where you are seeing this. Or can you point out the part on this diagram?

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  3. #3
    Senior Member Randall-in-Mpls's Avatar
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    I suspect he's referring to the needle in the vacuum piston that meters the secondary main jet. (Part #2 in the diagram. It mounts in part #8.)

    That retaining screw should be snug, but the needle will fit somewhat loosely. There should also be a white nylon plug over it (the uppermost bit in item #2.)


    R
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    1978 CX500 Standard - The Grub


    1981 GL500 Basket case -> AdventureWing (Dual-sport conversion in progress) - Part 1 - Part 2 - Part 3


    1979 CX500 Deluxe rolling chassis -> Hmm?

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  5. #4
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    Yes Randal that is what I'm talking about. Could this potentially cause bogging during heavy acceleration? You mention that the screw should be snug but the needle should have some play? I got the screw to be snug but the needle won't move.

  6. #5
    Super Moderator CXPHREAK's Avatar
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    Someone may have packed washers on the needle. Sometimes done to lift the needle. If they're in there throw them away {or they may fit the mixture screws} Needle jet wear often occurs if the needle can't 'float'.
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  7. #6
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    Thank you cxphreak. So the needle should have a little play? Ok I'll open it up and loosen the screw a little. I'll have to keep in mind that they fit for the mixture screw. Any tips on getting the plastic cap off the top? Seems to be on there pretty snug.

  8. #7
    Senior Member Randall-in-Mpls's Avatar
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    Don't just loosen the screw. Remove the shims.

    To remove the plastic plug, I have a narrow nozzle on my blowgun that fits closely inside the plug. A short blast of air results with the plug around the nozzle. Frankly, I hardly ever touch that plug, or the needle. But in this case, something is odd, and you need to determine what it is.


    R
    Last edited by Randall-in-Mpls; 02-05-2017 at 08:48 PM.
    10th Annual Minnesota Spring Ride and Barbecue - Saturday, May 13, 2017
    ----------------------------------------------------------------------------------------------------------------------

    1978 CX500 Standard - The Grub


    1981 GL500 Basket case -> AdventureWing (Dual-sport conversion in progress) - Part 1 - Part 2 - Part 3


    1979 CX500 Deluxe rolling chassis -> Hmm?

  9. #8
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    Randall, just so I'm understanding right the shims are the o-rings? Thank you for the response. Yea the only reason I was looking at it is because I'm fixing a cafe racer and have been having issues with bogging and I suspected the carbs. I opened them up and noticed that something seemed wierd that the needle wasnt held in but there was and screw in there that wasn't screwed in. How many 4 letter words do you guys let out when working on your keihin carbs lol?

  10. #9
    Senior Member Randall-in-Mpls's Avatar
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    The shims would be small washers around the needle. They hold the needle farther up in the vacuum piston. Some folks install them to make the secondary main jet open earlier in the piston's lift. I would think a larger jet would be more effective, but not being a tuner, I wouldn't know.

    On that last point, verify your jet sizes, too. Incorrect, or incorrectly placed, jets can cause your symptoms.


    R
    10th Annual Minnesota Spring Ride and Barbecue - Saturday, May 13, 2017
    ----------------------------------------------------------------------------------------------------------------------

    1978 CX500 Standard - The Grub


    1981 GL500 Basket case -> AdventureWing (Dual-sport conversion in progress) - Part 1 - Part 2 - Part 3


    1979 CX500 Deluxe rolling chassis -> Hmm?

  11. #10
    Senior Member Randall-in-Mpls's Avatar
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    Quote Originally Posted by CalfAyRaySir View Post
    I opened them up and noticed that something seemed wierd that the needle wasnt held in but there was and screw in there that wasn't screwed in.
    If the retaining screw was free under the plug, I suspect the PO loosened it to allow some float in the needle, and it backed itself out.

    Without the screw in place, the needle may not have been metering the jet at all. The resulting rich mixture could cause your bogging, too.


    R
    10th Annual Minnesota Spring Ride and Barbecue - Saturday, May 13, 2017
    ----------------------------------------------------------------------------------------------------------------------

    1978 CX500 Standard - The Grub


    1981 GL500 Basket case -> AdventureWing (Dual-sport conversion in progress) - Part 1 - Part 2 - Part 3


    1979 CX500 Deluxe rolling chassis -> Hmm?

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