Replace it if necessary.

Inner Tube Inspection
Spring Tension InspectionUpper 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 ...
Valve Guide Installation
Apply engine oil to the valve guide outer surface before
installation.
Heat the area around the valve guide hole to about 120
150°C (248 302°F).
NOTICE
Do not heat the cylinder head with a torch. This will
warp the cylinder head. Soak the cylinder head and
heat the oil.
Usin ...
Valve Removal
Remove the cylinder head (see Cylinder Head Removal).
Remove the valve lifter and shim.
Mark and record the valve lifter and shim locations so they
can be installed in their original positions.
Using the valve spring compressor assembly, remove the
valve.
Special Tools - Valve ...