Air cut-off valve function?
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  1. #1
    Senior Member obo515's Avatar
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    Feb 2017

    Air cut-off valve function?

    What is the function of these things? Do they allow air to lift the slides once the throttles are opened past idle? I just test started the bike in progress and one side (right) is not hitting. I have not removed carbs to clean yet but will as I have manifolds to change and I;m sure they need some work inside. It is getting fuel as it runs out drains when opened, spark is good at plug too and compression is present but untested for exact pressure as I it hasn;t run yet. I'm just curious if the slide wont lift if cut=off valve is stuck shut or ruptured. Thanx

  2. #2
    Senior Member reclinedrelic's Avatar
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    Aug 2010
    The air cut off valve is to prevent "popping" on deceleration.

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  3. #3
    Senior Member obo515's Avatar
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    Feb 2017
    OK, I'm guessing the problem is dirty carbs as the bike has not been running for some time. Why would this bike be any different than the other neglected rides I've had ,,,? The carbs are very similar to what was on the older CB/L 450, but I don't recall a cut-off valve on that type. I'll fix something else on bike while waiting for carb parts form Asia. Thanx

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  5. #4
    Super Moderator CXPHREAK's Avatar
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    Aug 2010
    South Australia
    CB 450 carbs are similar to these and didn't have an aircut valve. The carbs sprouted them for the 500T though. They're bastards too.

    The ACVs only richen the mixture on deceleration - as said - to eliminate popping and shouldn't prevent the bike from starting.

    The choke circuit draws through the idle circuit. If the bike has been a non runner for any length of time it is fairly likely to have a clogged idle circuit. Some of the drillings are .5 mm with 0018 thou in the idle jet and the main gallery for it looks like a broken dogleg.
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  6. #5
    Senior Member obo515's Avatar
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    Feb 2017
    I've always called the condition the "bad gas disease". Many cans of carb cleaner have been applied to fix, and the idle circuits are the worst! Just getting the pilot jet out can be trying, the proper screwdriver is a must. I always continue spraying cleaner through all passages until it comes out the idle holes, etc. Ordered carb kits and floats thru ebay, cheap but possibly coming on a slow boat from China,,,,

  7. #6
    Senior Member obo515's Avatar
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    Feb 2017
    Worst case possibility, if idle passages are corroded shut or plugged beyond carb cleaner is there any way other than getting another carb body to fix? We used to buy new Amal carbs on Brits when that occured as they were impossible to clear. Amals were relatively cheap and available, not like these Kehins. Also, how much corrosion on slide edge or inside bell is too much to seal properly on these carbs? I did remove the top of the offending unit and the edge had some white growth on it, removed it with fine steel wool and same with inside bell. Donor carbs on ebay from 50 to 175 a pair, and no guarantee they have good parts either.

  8. #7
    OCR is offline
    Senior Member OCR's Avatar
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    Sep 2010
    Groton NY
    A fine wire (telephone or cat5 stripped) works for flossing out the passages of the idle circuit.
    If you have or have access to an Ultrasonic cleaner, that will help immensely.
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