The vacuum port on the other side is for Vacuum Brake booster set up if you have it. The reason one hose goes inside the air cleaner box is to "draw clean
air" into the crankcase to purge the build up blow by gases through the PCV valve to the carburator base, which is relatively high vacuum. I suppose it would work without the vent as blowby gases allow flow to the carburator base, but you would not get the volume of air moving through as you would with the vent to the air cleaner. Blowby gases would then contaminate the oil more.
Originally Posted by ehansen007
Will it also work if you have it vented to the base of the carb or does it have to go into the air cleaner? Seems they both go the same spot which is back into the engine. My Holley Street avenger has PCV's on both sides of the base.