Positive Crankcase Ventilation: Description and Operation

Fig.14 - Dual Return Passage Crankcase Ventilation System:

Blowby vapor is returned to the combustion chamber through the carburetor and intake manifold. Vapors are separated in the condensation chamber and directed into the carburetor venturi or beneath the throttle plate depending on the amount of manifold vacuum. On some a drain tube is used to collect vapor that has condensed.

When the throttle valve is closed (idle) or partially open, blow-by vapor is returned to the intake manifold through both breather hoses A and B and passage one (fixed orifice). When the throttle valve is wide open, vacuum pressure in the air cleaner case rises and vacuum pressure at passage one (fixed orifice) decreases. Blowby vapor is drawn into the air cleaner case through vapor passage two, from the condensation and breather hose A. A small amount of vapor is returned through passage one (fixed orifice).