Unmodified Model A carburetors have a hole above the chamber where the float valve is that runs to the outside of the carburetor. This works great unless an air cleaner has been installed that has enough restriction to cause the carburetor to run rich. (Sort of like running with the choke half on.) Modern (and modified carburetors) have the hole lead to inside the carburetor intake just upstream of the choke plate. That way the air cleaner has no effect on the fuel/air ratio.
Bottom line: Use an air cleaner with very little restriction or use a modified carburetor.