Bumping this back to the top. How's the bike 2CV?
I think the plastic vent tubes are to carry the gas away from the air intake if you get grit under the float needle holding it open and causing the carb to flood and carry on flooding, thereby draining your gas tank onto the road, and thus achieving a slight decrease in the fire risk! That's what the otherwise excellent VM 32 Mikuni on my old BSA did to me once, before I fitted an in line fuel filter. Dell'Orto PHF's also have them, though they're black. Perhaps Mikuni think we just love pink spaghetti draped everywhere, part of the inscrutable Japanese sense of humor?A.
It's a vent.The fuel vapor in the tube is most likely condensing on the tube wall.I wouldn't worry about it.