The balance tube is located in the air horn in such a way that the tube is outside the choke valve when the valve is closed. During periods of choking, the balance tube acts as an outside vent to the float bowl, permitting a normal flow of fuel through the main nozzle.
When the choke has been moved to the open position the rush of air past the end of the balance tube tends to draw air from the float bowl causing a slight vacuum which retards flow of fuel to the nozzle.
The object of a balanced carburetor is to provide a constant air fuel ratio regardless of type, size, or condition of the air cleaner.
Should the air cleaner element become clogged, the capacity will be reduced but the air fuel mixture ratio will not be changed.
When rebuilding, or cleaning your carburetor check to make sure the balance tube is clean and not clogged with anything.