If the surface is corroded or if the blades are damaged, replace the impeller

Water Pump Impeller Disassembly/Assembly
Water Pump Housing DisassemblyDrive Chain
The drive chain slack and lubrication
must be checked each day before riding
the motorcycle and must be maintained
in accordance with the Periodic
Maintenance Chart for safety and to
prevent excessive wear. If the chain
becomes badly worn or maladjusted -
either too loose or too tight - the c ...
Water Pump Housing Disassembly
NOTICE
Do not damage the hole wall of the water pump
housing.
Insert a bar [A] into the pump housing [B], and hammer
evenly around the circumference of the mechanical seal
bottom [C].
Take the oil seal [A] out of the housing [B] with a hook [C].
...
Piston Wear Inspection
Measure the outside diameter [A] of each piston 5 mm
(0.20 in.) [B] up from the bottom of the piston at a right
angle to the direction of the piston pin.
If the measurement is under service limit, replace the piston.
Piston Diameter
Standard: 75.959 75.974 mm
(2.9905 2.9911 in.)
...