Yamaha YFM250XL(C) Service Manual page 121

Å
INSPECTION AND REPAIR
Press the valve through the valve guide
and onto the valve seat to make a clear
pattern.
Remove the valve from the cylinder head.
Measure the valve seat width a.
When the valve seat and valve face make
contact, bluing will be applied to the valve
face.
Valve seat width
Intake
Exhaust
If the valve seat width is too wide, too nar-
row, or seat has not centered. The valve
seat must be refaced.
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4.Reface:
Valve seat
Use 30˚, 45˚ and 60˚ Valve Seat Cutter.
Valve seat cutter set:
P/N. YM-91043-B
CAUTION:
Remove just enough material to achieve
satisfactory seat. When twisting cutter,
keep an even downward pressure to pre-
vent chatter marks.
Cut sections as follows
Section
A
B
C
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Valve seat refacing steps:
Å Valve face indicates that valve seat is cen-
tered on valve face but is too wide.
Valve seat cutter set
30˚ cutter To reduce valve seat
Use
lightly
60˚ cutter
4 - 23
ENG
0.9 ~ 1.1 mm
1.6 mm
(0.035 ~ 0.043 in)
(0.063 in)
Cutter
30˚
45˚
60˚
Desired result
width to 1.0 mm
(0.04 in)
Limit
4