This is definitley a vent on mine... ran it plugged, and the bike would not run. Opened, it ran like a champ. Someone's gotta have an answer as it's on all the Cali bikes. Anyone got a minute to look at their Bonnie carbs? This was originally connected to a canister which was part of the smog stuff. I'd hate to find out it's sucking air into the carbs unfiltered.
Even the Euro (French?) Thrux's seem to have it:

(From the Wiki pages)
Just found this on the WIki. Guess I'm not the only one. (from the how to instuctions on AI removal)
EVLS
16. Fit one end of the 5/16” hose to the crossover fuel bowl vent tube (this is a plastic tube in
between the carbs at about top center that has a 5/16 fitting in the middle) THIS
FITTING MUST BE VENTED and route this down behind the engine, cut a strip of the
white breather filter material and stuff it inside the tube to keep out debris,
17. Check your fuel tank vent hose (this hose is attached to the frame member behind the
carbs) and see if part of it separated (mine came apart just below the inline filter), if so, a
smaller un-ribbed piece of hose is what you are looking for to reattach; run this hose
downwards as far as it will reach. You may want to replace it with a slightly longer hose.
18. Now reassemble. Don’t forget to reattach the petcock hose and vent hose properly.
On the first try at the above, I plugged the fuel bowl vent tube (see #16) and the result was a
near disaster, as with the fuel floats unvented, the floats never floated and never stopped
flowing gasoline and gasoline was fuming off the headers before I noticed it, as I was trying
to get the bike running. Once the vent was opened up, everything was perfect.
[ This message was edited by: unitedrocker on 2007-06-05 02:57 ]