I just finished cleaning and reinstalling my carbs. This is the first time I've done it on my gs500. While I was putting everything back together, I noticed that there was an unconnected spot on the petcock for a vacuum hose which is supposed to attach to the left carb...but there was no hose attached.

It was running without this hose (though not well). What is this hose supposed to do?

that hose supplies vaccum to the petcock, so that it turns the fuel off when bike isn't running.  without the hose, you will have a vaccum leak on
the left carb, and most times horrible problems with the idle

heh thanks for the info. guess i'm gonna need that! that might explain why i had a hard time keeping idle.

Hard time getting fuel in general! Lol.. use 1/4 inch line, you'll need about a foot... but see if you can order it from Suzuki first. There is a 90 degree bend on the end that goes to the carb to prevent it from kinking... if it kinks it will run WORSE than without the hose. You also need clamps on it, fyi!

1/4 in and not 3/16"? i read on an oem site 5x9x350. im guess that's 5mm i.d.(?) and 350mm length which is ~3/16" id. or maybe 1/4" hose has 3/16" i.d. i always get hose/tube/pipe measurements messed up. vthe length is about a foot like you say.

i think i'll try getting just a generic hose from pep boys or something tonight so i can see if it works. i hate waiting for parts from oem dealers...i havent found one that doesn't take like 2 weeks to ship.

it's alive!

ok i bought a 3/16" hose and hooked it up. what a beast. the bike is running better than it ever has since i bought it. i think i messed up the tension on the throttle cable though because its idling really high and the tach is really slow to respond (not quite sure why this is the case)

anyway, a 3/16" i.d. hose is what fits snug for any curious readers.

There's a knob directly under the carb that you can adjust the idle with.

Jeremy's half right.  That tube applies vacuum to the diaphragm when the engine is running, which opens the diaphragm on the petcock to allow fuel to reach the float bowls.  It's a safety feature.  But yes, the tube must be connected or you'll have a vacuum leak and crapola idling.  Ride in the PRIME position.  Park it in ON or RESERVE.  You'll be happier if you do!    The PRIME position bypasses the diaphragm which is famous for not allowing enough fuel to flow.

If you ride it in the prime position and run the fuel out, you will not have a reserve because the prime setting takes fuel direct from the bottom of the tank (I think, maybe someone else can clarify).

That's what the trip odometer is for.  Reset it when you fill-up.  When it reaches 240 miles you have half a gallon left, unless you ride like a maniac.  In that case, re-fill when it reaches 200 miles