Positive Crankcase Ventilation (PCV) valves

[Tony Wheeler]

I'm not familiar with PCV valve design, but I'm guessing it's a bypass passage designed to provide a degree of crankcase ventilation at high vacuum, ie. under light throttle. In other words, a transition passage between idle and load conditions, just like carbies have for fuel metering. Obviously it would act as an air bleed at idle, but presumably it's small enough to be compensated by readjustment of the idle speed and mixture screws. Anyway it's obviously designed that way so it would be worth trying before changing the plunger.