Yamaha YZ250F(X) Owner's Service Manual page 55

ADJUSTING THE VALVE
CLEARANCE
• The valve clearance should be ad-
justed when the engine is cool to
the touch.
• The piston must be at Top Dead
Center (T.D.C.) on compression
stroke to check or adjust the valve
clearance.
1. Remove:
• Seat
• Fuel tank
Refer to "SEAT, FUEL TANK
AND SIDE COVERS" section in
the CHAPTER 4.
2. Remove:
• Spark plug
• Cylinder head cover
Refer to "CAMSHAFTS" section
in the CHAPTER 4.
3. Remove:
• Timing mark accessing screw "1"
• Crankshaft end accessing screw
"2"
• O-ring
4. Check:
• Valve clearance
→
Out of specification
Valve clearance (cold):
Intake valve:
0.10–0.15 mm
(0.0039–0.0059 in)
Exhaust valve:
0.17–0.22 mm
(0.0067–0.0087 in)
Checking steps:
a. Turn the crankshaft counterclock-
wise with a wrench.
b. Align the T.D.C. mark "a" on the
rotor with the align mark "b" on the
crankcase cover when piston is at
T.D.C. on compression stroke.
In order to be sure that the piston is at
Top Dead Center, the punch mark "c"
on the exhaust camshaft and the
punch mark "d" on the intake cam-
shaft must align with the cylinder
head surface, as shown in the illustra-
tion.
c. Measure the valve clearance "e"
using a feeler gauge "1".
Record the measured reading if the
clearance is incorrect.
Adjust.
5. Adjust:
• Valve clearance
Adjustment steps:
a. Remove the camshaft (intake and
exhaust).
Refer to "CAMSHAFTS" section
in the CHAPTER 4.
b. Remove the valve lifters "1" and
the pads "2".
• Place a rag in the timing chain
space to prevent pads from falling
into the crankcase.
• Identity each valve lifter and pad
position very carefully so that they
can be reinstalled in their original
place.
3-11
ENGINE
c. Select the proper pad using the
pad selecting table.
Pad Availabili-
Pad range ty: 25 incre-
ments
No. 1.20
Pads are avail-
120– mm–
able in 0.05 mm
No. 2.40
increments
240 mm
The thickness "a" of each pad is indi-
cated in hundredths of millimeters on
the pad upper surface.
d. Round off the last digit of the in-
stalled pad number to the nearest
increment.
Last digit of pad
Rounded valve
number
0, 1 or 2
4, 5 or 6
8 or 9
EXAMPLE:
Installed pad number = 148
Rounded off value = 150
Pads can only be selected in 0.05
mm increments.
e. Locate the rounded-off value and
the measured valve clearance in
the chart "PAD SELECTION TA-
BLE". The field where these two
coordinates intersect shows the
new pad number to use.
0
5
10