I just replaced the needle valve with a oem honda one and the carb is spotless. The float level is set to the factory specs of 16mm and it still leaks out the overflow.check the rubber tip on the float needle for damage and replace it if there is. there is also a chance that the seat has a film from old gas. you'll need to clean it if there is.
is there any signs of trash in the bottom of the carb when you last had it apart?