Water Testing; Checking For Cavitation; Jet Drive Disassembly; Jet Drive Removal - Mercury 25 EFI Service Manual

Jet Drive Outboard

Water Testing

Checking for Cavitation

The initial outboard height setting should be close to the optimum setting for the outboard. However, because of the hull design
of some boats, obstructions, or imperfections in the hull ahead of the water intake, adjustments may be required to prevent
cavitation at running speeds.
When operating the boat, the outboard driveshaft housing should be vertical, or tilted toward the boat, when planing to provide
a scooping angle on the water intake. Tilting the outboard out beyond a vertical position reduces the scoop angle and can
cause impeller slippage and cavitation.
IMPORTANT: If the angle of the boat transom does not allow the driveshaft housing to be positioned vertical, a wedge kit
should be installed behind the transom brackets to increase the tilt‑in angle.
NOTE: Slight cavitation in sharp turns and rough water is acceptable, but excessive cavitation is harmful to the outboard and
should be avoided.
Test run the boat. If cavitation occurs (air enters the pump), the first thing to try is lowering the outboard mounting height.
If cavitation still exists after lowering the outboard, it may be helpful to seek advice from the boat manufacturer.
Another option to further reduce cavitation is a rough water plate.
A rough water plate may be helpful in reducing cavitation when running in windy, rough water conditions where air is sucked
into the water intake when jumping waves. Install a 0.8 mm (1/32 in.) metal plate that extends from the hull bottom to the top of
the water intake housing. This plate tends to reduce air intake as well as reduce spray.

Jet Drive Disassembly

Jet Drive Removal

1. Disconnect the shift linkage.
2. Remove the six mounting bolts securing the water intake housing and remove the water intake housing.
b
3. Straighten the bent tabs on the nut retainer and remove the impeller nut.
Page 9A-12
a - Rough water plate
a
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© 2016 Mercury Marine
a - Mounting bolts
a
b - Water intake housing
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90-8M0105568 eng MARCH 2016