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Thread: Hose from carbs

  1. #1
    CB750 New Member barrage's Avatar
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    Hose from carbs

    Hi Everyone,

    Last year I bought a CB750K8. ( I originally bought one new in 78)

    Coming off the carbs are two hoses, one from the fuel tank and one disconnected (looks like broken from somewhere) that I cannot find any reference to. Can anyone tell me where it goes. I am guessing it might cause a vacuum leak and affect performance. See attached picture, the one indicated by red arrow is the one in question.

    Thanks

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  2. #2
    CB750 Guru amc49's Avatar
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    Probably a main vent joining all 4 carbs up. The only reason it would be vacuum would be to power a vacuum fuel valve and those did not appear for a few more years.

    You can plug it and see, bike will begin to run like crap with it closed. The later ones joined all vents into one pipe to easily control it for emissions.

    If bike is running fine then not what you think, a vacuum leak would show at idle instantly.

  3. #3
    Moderator dirtdigger's Avatar
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    Its just a vent. No vacuum lines on these.

  4. #4
    CB750 Addict Medyo Bastos's Avatar
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    iirc it goes to the gas cap vent. Notice the hole under your gas cap lid. There is an additional nipple (I like that word) under the tank. Any overfilled, or spilled fuel during fill up would drain down this tube...


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  5. #5
    CB750 New Member barrage's Avatar
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    Quote Originally Posted by amc49 View Post
    Probably a main vent joining all 4 carbs up. The only reason it would be vacuum would be to power a vacuum fuel valve and those did not appear for a few more years.

    You can plug it and see, bike will begin to run like crap with it closed. The later ones joined all vents into one pipe to easily control it for emissions.

    If bike is running fine then not what you think, a vacuum leak would show at idle instantly.


    HI AMC49,

    Thanks for the reply. There only seems to be 1 of these hoses but it could be the common vent for the carbs. Not sure where it would be vented to.. The air box maybe? There is a small hole close to the larger hose fitting, but it does not look like anything is broken off it. (In the end of the hose, it would seem a plastic fitting is there that was broken off somewhere.

    Bike seemed to run fine, although not great when cold. Might be an emissions thing that doesn't do much.

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