PROPULSION
6.32004 Four Stroke PWC Service Manual
Ventilation Troubleshooting
Ventilation results when air enters the pump inlet tract and is compressed by the impeller, causing a reduction
in thrust and an increase in engine RPM. Small leaks are often difficult to diagnose.
Some possible sources of air are:
1. An improperly sealed intake scoop is the most common cause of ventilation. Make sure the scoop has a com-
plete bead of Marine Grade silicone sealer all the way around without any gaps or pin holes.
2. Ride plate -- The ride plate must be completely sealed in the area around the intake scoop, and also in the area
where the hull meets the pump cavity.
3. Driveshaft bearing housing or through hull fitting -- If the drive shaft bearing carrier seals are worn or damaged,
air may be drawn into the pump from the engine compartment. Also inspect the through-hull fitting. The entire
circumference of the fitting must be sealed to prevent water from entering the hull, and air from entering the pump.
4. Incorrectly mounted pump. This can be caused by an improperly installed intake scoop or a misaligned pump
(there should be no detectable gap between pump housing and hull).
Cavitation Troubleshooting
Cavitation results when a low pressure area is created in the vicinity of the impeller blades, causing the surround-
ing water to boil or more accurately, the gas bubbles within the water implode with a destructive force. Cavitation
may be caused by cracked, damaged, bent or broken impeller blades, or an excessively worn impeller or housing
(check impeller clearance).
Ventilation or cavitation can drastically reduce the amount of output thrust produced by the pump. Evidence of
either problem may appear as burn marks on the impeller and stator vanes in the pump.
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