Installation manual

1-63
90-898305
Theory of Operation - 3 Cylinder Models
Air Induction Through Crankcase
Combustion air enters the cowl through holes located in the top aft end of the
cowl. The cowl liner directs this air to the bottom of the powerhead. This limits
the exposure of salt air to the components inside the engine cowl. Once inside
the cowl the air enters the plenum through the throttle shutter which is located
in the plenum assembly. The air then continues through the reed valves and
into the crankcase. The throttle shutter is actuated by the throttle shaft.
Mounted on a separate shaft is a throttle position sensor (TPS). This sensor
tells the propulsion control unit (PCM) the position of the throttle.
Air Compressor System
Air from inside the engine cowl is drawn into the compressor through an air
filter in the air attenuator. This attenuator acts like a muffler to quiet
compressor noise and contains a filter to prevent the ingestion of debris into
the compressor. The compressor is driven by a belt from a pulley mounted on
the flywheel and is automatically self-adjusted using a single idler pulley. This
air compressor is a single cylinder unit containing a connecting rod, piston,
rings, bearings, reed valves, and a crankshaft. The compressor is water
cooled to lower the temperature of the air charge and is lubricated by oil from
the engine oil pump assembly. As the compressor piston moves downward
inside the cylinder, air is pulled through the filter, reed valves and into the
cylinder. After the compressor piston changes direction, the intake reeds close
and the exhaust reeds open allowing compressed air into the hose leading to
the air/fuel rails.
The air/fuel rails contain two passages; one for fuel, the second for air. The air
passage is common between all the cylinders included in the rail. A hose
connects the rail air passage to the air compressor. An air pressure regulator
will limit the amount of pressure developed inside the air passages to
approximately [i.e. 648.1 kPa (94 psi) air vs 744.6 kPa (108 psi) fuel]. Air
exiting the pressure regulator is discharged through a muffler into the lower
cowling.
Notes
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