Topic: Carb- what's this hose for?? PIC

[BlackMax]

I was told it was some vacuum hose for an automatic bike (the guy claimed the carbs are off an auto.  I don't know if that's correct or not.).   Since my bike's not an auto, he said it just needs to be plugged off.    Can anyone provide the low-down on this one?  It's the hose with the bolt screwed into it.

Obviously, this is #1 carb.  The hose on the right is the fuel supply from the petcock.  

Thanks!

[TwoTired]

Looks like your standard carb bowl vent, to me.  It is needed to equalize the pressure inside to compensate for fuel level changes.  The stock bike routes that hose down behind the engine and in front of the swing arm.  The long hose discourages bugs from making the journey, as well as keeping water and dust from settling in the carb bowls.  If you plug that hose, you then rely on the individual carb bowl drain tubes for pressure equalization.  That is, if the carbs aren't overflowing. 

[BlackMax]

Looks like your standard carb bowl vent, to me.  It is needed to equalize the pressure inside to compensate for fuel level changes.  The stock bike routes that hose down behind the engine and in front of the swing arm.  The long hose discourages bugs from making the journey, as well as keeping water and dust from settling in the carb bowls.  If you plug that hose, you then rely on the individual carb bowl drain tubes for pressure equalization.  That is, if the carbs aren't overflowing.  

Just as I suspected......the person who said it was some vacuum was....um.....incorrect.

So, it simply keeps the bowl air pressure at 1 atmosphere.  Of course there are also vents in each bowl, but they could get clogged.  

Even though it's attached to carb #1 only?  The vent affects all 4 bowls?  A clog would tend to discourage re-filling of the bowls, due to atmospheric pressure, right?

[TwoTired]

If it's like the carbs on my 750s (PD) there are rubber couplers between carbs forming a log manifold and linking all the carb bowl chambers together.
One hose therefore feeds four carbs.  As mentioned, the carb bowl stand pipes also can contribute to bowl equalization, provided they aren't full of fuel or have their tube ends plugged by mud daubers. (Need to check that if the bike stands still outside for a few days.)