[RichardJ]

>>Crankcase gases are never intended to be drawn through the carb, and in fact this only happens on an engine with excessive blowby.<<

During WOT (wide open throttle) manifold vacuum is at a minimum and at the same time, blow-by is at its maximum. During this event, pressure above the carb, as to say, between the mouth of the carb and the air cleaner, will be low and the increased pressure in the crankcase, will cause crankcase fumes to travel into the air cleaner and through the carb.

During idle and cruise, the high manifold vacuum pulls the filtered air from the air cleaner, into the crankcase, out of the crankcase, through the PCV Valve, then into the intake manifold.
Not all carbs have a vacuum port for the PVC System. Sometimes a vacuum port in the intake manifold is provided for the PCV System.

When you have a valve cover breather, you have what is called an OPEN PCV System. During WOT, crankcase fumes are expelled out into the atmosphere.
A CLOSED PCV System is fully contained and does not expel crankcase pollutants out into the atmosphere.