Yamaha YZ250F(E), Owner'S Service Manual

Page: 218 / 356

VALVES AND VALVE SPRINGS
5-25
TIP
After replacing the valve guide, reface the valve
seat.
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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 margin thickness D “a”
Out of specification

Replace the valve.
6. 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 oil seal.
EAS1SM1222 CHECKING THE 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 C “a”
Out of specification

Replace the cylinder
head.
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a. Apply Mechanic’s blueing dye (Dykem) “b”
onto the valve face.
Intake
Valve guide remover (ø5.0)
90890-04097
YM-04097
Valve guide installer (ø5.0)
90890-04098
YM-04098
Valve guide reamer (ø5.0)
90890-04099
YM-04099
Exhaust
Valve guide remover (ø4.5)
90890-04116
YM-04116
Valve guide installer (ø4.5)
90890-04117
YM-04117
Valve guide reamer (ø4.5)
90890-04118
YM-04118
Valve margin thickness D (intake)
1.20 mm (0.0472 in)
Valve margin thickness D (ex-
haust)
0.85 mm (0.0335 in)
Valve stem runout
0.010 mm (0.0004 in)
Valve seat width C (intake)
0.90–1.10 mm (0.0354–0.0433 in)
Valve seat width C (exhaust)
0.90–1.10 mm (0.0354–0.0433 in)