It's a little bit of "tech", but here's the whole skinny on the vents:
1. The throat of the carbs is at low air pressure (some call it "vacuum", bit it is not quite that low) from the piston occasionally sucking thru it.
2. The tiny orifices in the bell area of the carbs, which feed air to the emulsifier tubes above the mainjet and idle jet, are at slightly less of a low pressure, being in a still-air zone where the flowing air is above them, not right on them. This higher pressure is used to help push the aerated fuel in the emulsifer tubes up into the throat of the carb.
3. The bowl vents are at atmospheric pressure, higher than the pressure inside the airbox or the emlusifiers (the airbox is slightly lower pressure than outside air or bowl vent air because it is slightly restricted, on purpose, by the air filter). These vents cause the bowls to be higher pressure so they can push the fuel into the emulsifiers, from the bowl thru the mainjet and idle jet.
The differences between these pressures are less than 0.025 PSI between the bowl and bell area, and less than 0.05 PSI between the emulsifiers and the carb throat at the venturis. So, it does not take much to mess this relationship up: by using long bowl vent hoses to a quiet area (like under the seat or behind the airbox) you can ensure high bowl pressures. Also, by avoiding the use of "pod" air filters, you can preserve the tiny difference between the throat and emulsifiers. Without these tiny differences, the carbs don't mix, and the bike suffers...a lot.
