1977 Suzuki GS750 vacuum lines

Discussion in 'Motorbike Technical Discussion' started by robert.korn, Oct 16, 2005.

  1. robert.korn

    robert.korn Guest

    I just picked up a 1977 Suzuki GS750. There are vacuum lines connected
    to the bottom and top of all the carbs that are going nowhere. I can't
    find anywhere in the Clymer manual where they go. Is there someplace I
    can find a diagram, or does anyone know???
    robert.korn, Oct 16, 2005
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  2. There is probably actually only *one* vacuum hose. It goes from one
    carburetor to the back of the petcock to operate the automatic shutoff
    feature of the petcock.

    The fuel line (or lines) probably go to one or two "T" fittings between
    the #1 and #2 carbs and the #3 and #4 carbs.

    The hoses pointed up are probably float bowl vent tubes and the
    diagrams on www.partsfish.com and www.bikebandit.com don't say where
    they go, but the only hose fitting I can see on the air box is for the
    large 5/8th's inch hose that goes from the oil mist separator on the
    valve cover to the airbox.

    The float bowl vents on my Suzuki GS-1100 don't go anywhere either, all
    that matters is that they don't get pinched off when you install the
    gas tank.

    On later motorbikes equipped with charcoal canisters and other
    evaporative control devices the float bowl vents would go to the air
    box, but if you don't have a charcoal canister the only thing you have
    to do is route them so they won't get pinched shut.

    The hoses pointed down are float bowl overflow tubes. They dump
    overflow gas out of the float bowls onto the ground. Run them down the
    back side of the engine between the swingarm and the frame.
    krusty kritter, Oct 17, 2005
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  3. robert.korn

    popkorn Guest

    I posted a picture at <a
    There are three hoses that run between each of the 4 carbs, and then
    each carb also has a hose running from the bottom. Thanks for your
    reply, the original owner removed the OEM airbox and these hoses are
    just lying there, I was assuming they were vents but I wasn't sure.
    There is on larger nipple on the top of the intake that looked like a
    vacuum port, but I believe that is just a breather that goes to the
    large nipple on top of the OEM airbox. There is also a large nipple at
    the base of the OEM airbox, I'm guessing these vent hoses from the carbs
    were plumbed there. Let me know what you think after seeing the pics,
    popkorn, Oct 17, 2005
  4. popkorn wrote:

    Yes, those hoses are shown on the 1978 GS750C carburetor diagrams at
    www.partsfish.com and at www.bikebandit.com. I can't find a GS750B
    diagram anywhere.
    Yes, that is for the breather hose from the oil mist separator to the
    airbox. The oil mist separator works on the principle that the wire
    mesh inside is cooler than the oil vapors coming out of the crankcase,
    so the oil mist condenses and drips back into the crankcase and all
    that is supposed to get to the airbox is fumes.
    If the nipple you're talking about is about 3/8th's of an inch in
    diameter it's probably for an appendix.

    The oil mist separator just doesn't work all that well, and some oil
    droplets will condense in the bottom of the air box. Air boxes will
    often have an appendix hooked to the bottom of the air box. The
    appendix is a rubber tube with a closed end and the motorbike owner is
    somehow just supposed to know to take the appendix off and dump out the
    oil occasionally before it gets the air cleaner all oil soaked.

    When I removed the airbox from my Suzuki to use K&N filters, I had no
    place to run the oil breather hose to, and the rules of the racing
    organization demanded that I install an oil catch bottle. So I ran the
    breather hose all the way down to the bottom of the catch bottle.

    Everything worked fine until I got a quarter of an inch of oil in the
    bottom of the catch bottle. That sealed off the oil breather hose so
    air couldn't pass through it. Pressure built up in the crankcase and
    the oil fume pressure had no place to go, so it forced oil out past the
    rubber seal on the starter and the cavity that the starter sits in
    filled up with oil.

    You could run into that same kind of problem by hooking the float bowl
    vents to the bottom of the air box. You would have mysterious fuel
    starvation problems if the hose got blocked with condensed oil vapors.

    If you can hook the float bowl vents up to the air box closer to the
    top, that would be better, but not mandatory. If you can't find any
    manual that shows the float bowl vent hoses going to the air box, I
    wouldn't recommend hooking them to the air box at all. The main reason
    for doing that would be for evaporative emissions control, not for

    The fuel level in the float bowls might rise a little too high when the
    engine was running, but the lowered air pressure inside the float bowls
    would not push fuel up through the idle jets as easily. One effect
    compensates for the other though.

    The engine might run a little leaner at idle. There should be a pilot
    air adjustment screw on the side of each carburetor to adjust the idle
    Turning the screws clockwise makes the mixture richer.

    We were also discussing the pilot gas screws which only GS-750B
    carburetors seem to have about a week ago.

    They are underneath the carburetor, accessible through a hole in the
    float bowl. It's best to leave them alone if you can adjust mixture
    adequately by adjusting the pilot air screws.
    krusty kritter, Oct 17, 2005
  5. robert.korn

    popkorn Guest

    guessing > these vent hoses from the carbs were plumbed there. Let me
    know what

    Thanks for all your help!
    popkorn, Oct 17, 2005
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