Yamaha MBK MW125 2014, Service Manual

Page: 227 / 380

VALVES AND VALVE SPRINGS
5-24
3. Eliminate:
• Carbon deposits
(from the valve face and valve seat)
4. Check:
• Valve face
Pitting/wear
→
Grind the valve face.
• Valve stem end
Mushroom shape or diameter larger than the
body of the valve stem
→
Replace the valve.
5. Measure:
• Valve stem runout
Out of specification
→
Replace the valve.
TIP
• When installing a new valve, always replace
the valve guide.
• If the valve is removed or replaced, always re-
place the valve stem seal.
EAS30285
CHECKING THE VALVE SEATS
The following procedure applies to all of the
valves and valve seats.
1. Eliminate:
• Carbon deposits
(from the valve face and valve seat)
2. Check:
• Valve seat
Pitting/wear
→
Replace the cylinder head.
3. Measure:
• Valve seat width “a”
Out of specification
→
Replace the cylinder
head.
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a. Apply blue layout fluid “b” onto the valve face.
b. Install the valve into the cylinder head.
c. Press the valve through the valve guide and
onto the valve seat to make a clear impres-
sion.
d. Measure the valve seat width.
TIP
Where the valve seat and valve face contacted
one another, the blue layout fluid will have been
removed.
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4. Lap:
• Valve face
• Valve seat
TIP
After replacing the cylinder head or replacing the
valve and valve guide, the valve seat and valve
face should be lapped.
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a. Apply a coarse lapping compound “a” to the
valve face.
Valve stem runout
0.010 mm (0.0004 in)
Valve seat contact width (intake)
0.90–1.20 mm (0.0354–0.0472 in)
Limit
1.6 mm (0.06 in)
Valve seat contact width (ex-
haust)
0.90–1.20 mm (0.0354–0.0472 in)
Limit
1.6 mm (0.06 in)