Crankcase Ventilation Systems: Operation

When the engine is idling or at part-throttle operation, blow-by vapor is returned to the intake manifold through breather hoses A and B, and through the metered orifice. When the throttle valve is wide open, vacuum decreases at the orifice and increases in the air cleaner. Vapors are then drawn through the air cleaner into the carburetor. A small amount of vapors still enter through the orifice.