1977 Suzuki GS750 vacuum lines

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???

 

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.

 

I posted a picture at <a
href="http://www.kornfamily.org/pics/carb.jpg">www.kornfamily.org/pics/c
arb.jpg</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,
thanks!

 

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
performance.

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
mixture.
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.

 

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

Thanks for all your help!