Replace it if necessary.

Inner Tube Inspection
Spring Tension InspectionRear Wheel Removal
Raise the rear wheel off the ground with the stand [A].
Remove:
Cotter Pin [A]
Rear Axle Nut [B]
Washer [C]
Rear Axle [D] (from Right Side)
Remove the rear wheel rotation sensor from the caliper
bracket (see Rear Wheel Rotation Sensor Removal in the
Brakes chapter).
...
Piston Installation
Apply molybdenum disulfide oil solution to the oil ring expander,
and install the oil ring expander [A] in the bottom
piston ring groove so the ends [B] not butt together
Applymolybdenum disulfide oil solution to the oil ring steel
rails, and install the oil ring steel rails, one above th ...
Starter Motor Assembly
Install the positive brush assy [A] and the starter motor
terminal [B] as shown in the figure.
Replace the O-ring [C] with a new one, and install it.
Install:
Brush Springs [A]
Negative Brush Assy [B]
Turn the stopper side [C] to inside.
Tighten:
Torque - Brush Holder S ...