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

Water Pump Impeller Disassembly/Assembly
Water Pump Housing DisassemblyOperating Procedures
Clean the seat area carefully.
Coat the seat with machinist’s dye.
Fit a 45° cutter into the holder and slide it into the valve
guide.
Press down lightly on the handle and turn it right or left.
Grind the seating surface only until it is smooth.
NOTICE
Do not grind the seat too ...
Periodic Maintenance Chart
The scheduled maintenance must be done in accordance with this chart to keep
the motorcycle in
good running condition. The initial maintenance is vitally important and must
not be neglected.
Periodic Inspection
#: Service more frequently when operating in severe conditions; dusty, ...
Upper Crankcase Assembly
When installing the breather plate, replace the gasket [A]
with a new one.
Apply a non-permanent locking agent to the threads of
the bolts [A] and tighten them.
Torque - Breather Plate Bolts: 9.8 N·m (1.0 kgf·m, 87 in·lb)
Install the fitting [A] until it is bottomed ...