Yamaha RS90K Manuel D'atelier page 484

VALVES AND VALVE SPRINGS
• Install the valve into the cylinder head.
• Turn the valve until the valve face and valve
seat are evenly polished, then clean off all of
the lapping compound.
NOTE:
For the best lapping results, lightly tap the valve
seat while rotating the valve back and forth
between your hands.
• Apply a fine lapping compound to the valve face
and repeat the above steps.
• After every lapping procedure, be sure to clean
off all of the lapping compound from the valve
face and valve seat.
• Apply Mechanic's blueing dye (Dykem) b onto
the valve face.
• Install the valve into the cylinder head.
• Press the valve through the valve guide and
onto the valve seat to make a clear impression.
• Measure the valve seat width c again. If the
valve seat width is out of specification, reface
and lap the valve seat.
11. Measure:
• Valve spring free length a
Out of specification → Replace the valve
spring.
12. Measure:
• Compressed valve spring force a
Out of specification → Replace the valve
spring.
b Installed length
5-33
ENG
Valve spring free length (intake and
exhaust):
39.73 mm (1.56 in)
<Limit>: 37.7 mm (1.48 in)
Compressed valve spring force
(installed):
136 ~ 158 N at 33 mm
(13.9 ~ 16.1 kg at 33 mm,
30.6 ~ 35.5 lb at 1.30 in)