The crankcase breather system relieves crankcase pressure produced by the downstroke of the pistons. Crankcase vapors are then directed into the intake air stream to be burned during normal combustion. Burning crankcase vapors eliminates the pollutants normally discharged from the crankcase.
See Figure 3-7. As pistons push downward, displaced air in the crankcase is vented through the crankshaft roller bearing into the cam compartment. The air then flows up the pushrod covers (1) into the rocker housing. The moving air absorbs a small amount if oil vapor as it travels through the engine.
The oil/air vapor rushes under the rocker arm support plate and passes through an opening at the bottom of the plate to enter the breather baffle compartment (2).
In the baffle compartment, the flow of air passes upward through the oil filter gauze, where the oil is removed from the air. Two pin holes in the rocker arm support plate allow the separated oil to drain back into the crankcase.
Passing through the oil filter gauze, the vapor passes through the umbrella valve (3) into the breather compartment. The umbrella valve only allows air to be vented one way.
In the breather compartment, air flows downward through holes aligned in the breather baffle, rocker arm support plate and rocker housing. Exiting the rocker housing, air enters a passageway cast into the top of the cylinder head. Proper orientation of the rocker housing gasket is critical for effective sealing of this passageway.
Air flows through the cylinder head passageway and through a passage in the air cleaner backplate bolt (4). It passes through a breather tube (5) into the air filter element. It then joins with the intake air stream and is burned during normal combustion.
NOTE
Always connect breather tubes. Loose or detached tubes vent crankcase gases into the atmosphere which violates emissions standards.
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