light YAMAHA YZ250F 2008 User Guide

4-4
EXHAUST PIPE AND SILENCER
CHECKING THE SILENCER AND
EXHAUST PIPE
1. Inspect:•Gasket "1"
Damage
→Replace.
CHANGING THE SILENCER FIBER
1. Remove: • Rivet (front) "1"
• Inner pipe "2"
Take care not to damage the rivet
fitting holes (ø4.9 mm) "a" in re-
moval.
Pull out the inner pipe while lightly
tapping the stay "b" on the silencer
using a soft hammer.
2. Replace:•Fiber "1"
3. Install: •Inner pipe "1"
•Rivet (front) "2"
• Apply heat resistant sealant along the plate edge "a" on the inside of
the silencer and also along the si-
lencer edge "b" as shown.
• Take care not to allow the fiber out
of place when installing the inner
pipe.
INSTALLING THE SILENCER AND
EXHAUST PIPE
1. Install:•Gasket
• Exhaust pipe "1"
• Nut (exhaust pipe) "2"
First, temporarily install both nuts,
then tighten either of these nuts to 13
Nm (1.3 m•kg, 9.4 ft•lb) and the other
to 20 Nm (2.0 m•kg, 14 ft•lb), and
then come back to the first one and
retighten it to 20 Nm (2.0 m•kg, 14
ft•lb).
2. Install:• Silencer clamp "1"
• Gasket "2"
• Silencer "3"
•Washer "4"
• Bolt (silencer) "5"
• Install the gasket with its meshed area "a" toward the exhaust pipe
side.
• The gasket should be installed ac- cording to the dimension shown.
Nut (exhaust pipe): 20 Nm (2.0 m•kg, 14
ft•lb)
Silencer clamp: 16 Nm (1.6 m•kg, 11
ft•lb)
Bolt (silencer): 30 Nm (3.0 m•kg, 22
ft•lb)
New

4-12
CARBURETOR
9. Install:• Push rod "1"
While holding down the lever 1 "2", in-
sert the push rod farthest into the car-
buretor.
10. Install:• Starter jet "1"
• Pilot jet "2"
• Spacer "3"
• Needle jet "4"
•Main jet "5"
11. Install: • Needle valve "1"
•Float "2"
• Float pin "3"
• After installing the needle valve to
the float, install them to the carbure-
tor.
• Check the float for smooth move- ment.
12. Install:
• Pilot screw "1"
• Spring "2"
•Washer "3"
• O-ring "4"
Note the following installation
points:
a. Turn in the pilot screw until it is lightly seated.
b. Turn out the pilot screw by the
number of turns recorded before
removing.
13. Install:• O-ring
•Leak jet "1"
• Float chamber "2"
• Bolt (float chamber) "3"
• Cable holder (throttle stop screw cable) "4"
• Hose holder (carburetor breather hose) "5"
14. Install: • Diaphragm (accelerator pump) "1"
• Spring "2"
• O-ring "3"
• Accelerator pump cover "4"
• Hose holder (drain hose) "5"
• Screw (accelerator pump cover)
"6"
Install the diaphragm (accelerator
pump) with its mark "a" facing the
spring.
15. Install:•Jet needle "1"
•Collar "2"
• Spring "3"
• Needle holder "4"
• Throttle valve plate "5"
To throttle valve "6".
16. Install: • Throttle valve assembly "1"
• Screw (throttle shaft) "2"
Install the valve lever rollers "3" into
the slits "a" of the throttle valve.
17. Install:• O-ring "1"
• Valve lever housing cover "2"
• Bolt (valve lever housing cover) "3"
Pilot screw (example): 2-3/8 turns out

4-13
CARBURETOR
18. Install:• Carburetor breather hose "1"
Install the carburetor breather hoses
to the carburetor so that the hoses do
not bend near where they are in-
stalled.
ADJUSTING THE ACCELERATOR
PUMP TIMING
Adjustment steps:
In order for the throttle valve height
"a" to achieve the specified value,
tuck under the throttle valve plate "1"
the rod "2" etc. with the same outer di-
ameter as the specified value.
a. Fully turn in the accelerator pump adjusting screw "3".
b. Check that the link lever "4" has free play "b" by pushing lightly on
it.
c. Gradually turn out the adjusting screw while moving the link lever
until it has no more free play.
INSTALLING THE CARBURETOR
1. Install:
• Carburetor joint "1" 2. Install:
• Carburetor "1"
Install the projection "a" between the
carburetor joint slots.
3. Install:• Hot starter plunger "1"
4. Tighten: • Bolt (carburetor joint) "1"
• Bolt (air filter joint) "2"
5. Install: • Throttle cable (pull) "1" • Throttle cable (return) "2"
6. Adjust: • Throttle grip free playRefer to "ADJUSTING THE
THROTTLE CABLE FREE PLAY"
section in the CHAPTER 3.
7. Install: • Throttle cable cover "1"
• Bolt (throttle cable cover) "2"
8. Install: • Throttle position sensor lead cou-
pler "1"
•Clamp "2" Refer to "CABLE ROUTING DIA-
GRAM" section in the CHAPTER
2.
Throttle valve height:
0.8 mm (0.031 in)
Carburetor joint: 10 Nm (1.0 m•kg, 7.2
ft•lb)
Hot starter plunger:2 Nm (0.2 m•kg, 1.4
ft•lb)
Bolt (carburetor joint): 3 Nm (0.3 m•kg, 2.2
ft•lb)
Bolt (air filter joint): 3 Nm (0.3 m•kg, 2.2
ft•lb)
Throttle cable (pull): 4 Nm (0.4 m•kg, 2.9
ft•lb)
Throttle cable (return):11 Nm (1.1 m•kg, 8.0
ft•lb)
Bolt (throttle cable cov-
er): 4 Nm (0.4 m•kg, 2.9
ft•lb)

4-17
CAMSHAFTS
4. Measure:• Camshaft-to-cap clearanceOut of specification
→Measure
camshaft outside diameter.
Measurement steps:
a. Install the camshaft onto the cylin-
der head.
b. Position a strip of Plastigauge
®
"1" onto the camshaft.
c. Install the clip, dowel pins and camshaft caps.
• Tighten the bolts (camshaft cap) in a crisscross pattern from innermost
to outer caps.
• Do not turn the camshaft when measuring clearance with the Plas-
tigauge
®.
d. Remove the camshaft caps and measure the width of the Plasti-
gauge
® "1".
5. Measure:• Camshaft outside diameter "a"Out of specification
→Replace
the camshaft.
Within specification
→Replace
camshaft case and camshaft
caps as a set. CHECKING THE CAMSHAFT
SPROCKET
1. Inspect:
• Camshaft sprocket "1"
Wear/damage
→Replace the
camshaft assembly and timing
chain as a set.
CHECKING THE
DECOMPRESSION SYSTEM
1. Check: • Decompression system
Checking steps:
a. Check that the decompression mechanism cam "1" moves
smoothly.
b. Check that the decompression
mechanism cam lever pin "2"
projects from the camshaft.
CHECKING THE TIMING CHAIN
TENSIONER
1. Check:• While pressing the tensioner rod
lightly with fingers, use a thin
screwdriver "1" and wind the ten-
sioner rod up fully clockwise.
• When releasing the screwdriver by pressing lightly with fingers,
make sure that the tensioner rod
will come out smoothly.
• If not, replace the tensioner as- sembly. INSTALLING THE CAMSHAFT
1. Install:
• Exhaust camshaft "1"
• Intake camshaft "2"
Installation steps:
a. Turn the crankshaft counterclock- wise with a wrench.
• Apply the molybdenum disulfide oil
on the camshafts.
• Apply the engine oil on the decom- pression system.
• Squeezing the decompression le- ver allows the crankshaft to be
turned easily.
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.
Camshaft-to-cap clear-
ance:
0.028–0.062 mm
(0.0011–0.0024 in)
:0.08 mm (0.003
in)
Bolt (camshaft cap): 10 Nm (1.0 m•kg, 7.2
ft•lb)
Camshaft outside diame-
ter: 21.959–21.972 mm
(0.8645–0.8650 in)

4-18
CAMSHAFTS
c. Fit the timing chain "3" onto both camshaft sprockets and install the
camshafts on the cylinder head.
The camshafts should be installed
onto the cylinder head so that the
punch mark "c" on the exhaust cam-
shaft and the punch mark "d" on the
intake camshaft must align with the
cylinder head surface, as shown in
the illustration.
Do not turn the crankshaft during
the camshaft installation. Damage
or improper valve timing will re-
sult.
d. Install the clips, camshaft caps "4" and bolts (camshaft cap) "5".
• Before removing the clips, cover the cylinder head with a clean rag to
prevent the clips from into the cylin-
der head cavity.
• Apply the molybdenum disulfide oil
on the thread of the bolts (camshaft
cap).
• Tighten the bolts to the specified
torque in two or three steps in the
proper tightening sequence as
shown.
The bolts (camshaft cap) must be
tightened evenly, or damage to the
cylinder head, camshaft caps, and
camshaft will result.
2. Install:• Timing chain tensioner
Installation steps:
a. While pressing the tensioner rod
lightly with fingers, use a thin
screwdriver and wind the tension-
er rod up fully clockwise.
b. With the rod fully wound and the chain tensioner UP mark "a" fac-
ing upward, install the gasket "1"
and the timing chain tensioner "2",
and tighten the bolt "3" to the
specified torque.
c. Release the screwdriver, check the tensioner rod to come out and
tighten the gasket "4" and the cap
bolt "5" to the specified torque.
3. Turn:
• CrankshaftCounterclockwise several turns.
4. Check: • Rotor T.D.C. markAlign with the crankcase align
mark.
• Camshaft match marks Align with the cylinder head sur-
face.
Out of alignment
→Adjust. 5. Install:
• Timing mark accessing screw "1"
• Crankshaft end accessing screw "2"
6. Install: • Timing chain guide (top side) "1"
• Cylinder head cover gasket "2"
• Cylinder head cover "3"
• Bolt (cylinder head cover) "4"
Apply the sealant on the cylinder
head cover gasket.
7. Install:• Cylinder head breather hose
• Spark plug
Bolt (camshaft cap):
10 Nm (1.0 m•kg, 7.2
ft•lb)
Bolt (timing chain ten-
sioner):10 Nm (1.0 m•kg, 7.2
ft•lb)
Tensioner cap bolt: 7 Nm (0.7 m•kg, 5.1
ft•lb)
Bolt (cylinder head cov-
er):
10 Nm (1.0 m•kg, 7.2
ft•lb)
YAMAHA Bond No. 1215
(ThreeBond
® No. 1215):
90890-85505
Spark plug: 13 Nm (1.3 m•kg, 9.4
ft•lb)

4-23
VALVES AND VALVE SPRINGS
4. Measure:• Margin thickness "a"Out of specification
→Replace.
5. Measure: • Runout (valve stem)Out of specification
→Replace.
• When installing a new valve always
replace the guide.
• If the valve is removed or replaced
always replace the oil seal.
6. Eliminate:• Carbon deposits(from the valve face and valve
seat)
7. Inspect: •Valve seat
Pitting/wear
→Reface the valve
seat.
8. Measure:
• Valve seat width "a"Out of specification
→Reface the
valve seat.
Measurement steps:
a. Apply Mechanic's blueing dye
(Dykem) "b" to 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 pattern.
d. Measure the valve seat width. Where the valve seat and valve
face made contact, blueing will
have been removed.
e. If the valve seat is too wide, too
narrow, or the seat is not cen-
tered, the valve seat must be re-
faced.
9. Lap:• Valve face
• Valve seat
After refacing the valve seat or re-
placing the valve and valve guide, the
valve seat and valve face should be
lapped.
Lapping steps:
a. Apply a coarse lapping compound to the valve face.
Do not let the compound enter the
gap between the valve stem and
the guide.
b. Apply molybdenum disulfide oil to the valve stem. c. Install the valve into the cylinder
head.
d. Turn the valve until the valve face and valve seat are evenly pol-
ished, then clean off all of the
compound.
For best lapping results, lightly tap
the valve seat while rotating the valve
back and forth between your hands.
e. Apply a fine lapping compound to the valve face and repeat the
above steps.
After every lapping operation be sure
to clean off all of the compound from
the valve face and valve seat.
f. Apply Mechanic's blueing dye
(Dykem) to the valve face.
g. Install the valve into the cylinder head.
h. Press the valve through the valve guide and onto the valve seat to
make a clear pattern.
i. Measure the valve seat width again. If the valve seat width is out
of specification, reface and relap
the valve seat.
CHECKING THE VALVE SPRINGS
1. Measure:• Valve spring free length "a"Out of specification
→Replace.
Margin thickness:
Intake:
0.8 mm (0.0315 in)
Exhaust: 0.7 mm (0.0276 in)
Runout limit: 0.01 mm (0.0004 in)
Valve seat width: Intake:0.9–1.1 mm (0.0354–
0.0433 in)
:1.6 mm
(0.0630 in)
Exhaust:
0.9–1.1 mm (0.0354–
0.0433 in)
:1.6 mm
(0.0630 in)
Free length (valve
spring): Intake:37.81 mm (1.49 in)
: 36.81 mm
(1.45 in)
Exhaust:
37.54 mm (1.48 in)
: 36.54 mm
(1.44 in)

4-24
VALVES AND VALVE SPRINGS
2. Measure:• Compressed spring force "a"Out of specification
→Replace.
b. Installed length
3. Measure:
• Spring tilt "a"Out of specification
→Replace.
CHECKING THE VALVE LIFTERS
1. Inspect: • Valve lifterScratches/damage
→Replace
both lifters and cylinder head. INSTALLING THE VALVES
1. Apply:
• Molybdenum disulfide oilOnto the valve stem and valve
stem seal.
2. Install: •Valve "1"
• Valve spring seat "2"
• Valve stem seal "3"
• Valve spring "4"
• Valve spring retainer "5"
To cylinder head.
• Make sure that each valve is in-stalled in its original place, also re-
ferring to the painted color as
follows.
Intake (middle) "a": Orange
Intake (right/left) "b": Green
Exhaust "c": Purple
• Install the valve springs with the
larger pitch "d" facing upward.
e. Smaller pitch
3. Install:
• Valve cotter
While compressing the valve spring
with a valve spring compressor "1" in-
stall the valve cotters.
4. To secure the valve cotters onto the valve stem, lightly tap the
valve tip with a piece of wood.
Hitting the valve tip with excessive
force could damage the valve.
5. Install:• Adjusting pad "1"
• Valve lifter "2"
• Apply the molybdenum disulfide oil on the valve stem end.
• Apply the engine oil on the valve lift- ers.
• Valve lifter must turn smoothly when rotated with a finger.
• Be careful to reinstall valve lifters
and pads in their original place.
Compressed spring
force: Intake:99–114 N at 29.13 mm
(9.9–11.4 kg at 29.13
mm, 22.27–25.57 lb at
1.15 in)
Exhaust:
126–144 N at 29.30
mm (12.6–14.4 kg at
29.30 mm, 28.44–
32.41 lb at 1.15 in)
Spring tilt limit: Intake:
2.5°/1.7 mm (0.067 in)
Exhaust: 2.5°/1.6 mm (0.063 in)
Valve spring compres-
sor:
YM-4019/90890-04019

4-58
CRANKCASE AND CRANKSHAFT
DISASSEMBLING THE
CRANKCASE
1. Separate:• Right crankcase
• Left crankcase
Separation steps:
a. Remove the crankcase bolts "1", hose guide "2" and clutch cable
holder "3".
Loosen each bolt 1/4 of a turn at a
time and after all the bolts are loos-
ened, remove them.
b. Remove the right crankcase "4".
• Place the crankcase with its left half
downward and split the right half
apart while lifting it horizontally by
lightly tapping the projection "a" on
it using a soft hammer.
• When splitting it, leave the crank-
shaft and transmission with the left
half.
Use soft hammer to tap on the
case half. Tap only on reinforced
portions of case. Do not tap on
gasket mating surface. Work slow-
ly and carefully. Make sure the
case halves separate evenly. If the
cases do not separate, check for a
remaining case bolt or fitting. Do
not force.
c. Remove the dowel pins and O- ring.
REMOVING THE CRANKSHAFT
1. Remove:• Crankshaft "1"Use the crankcase separating
tool "2".
Do not use a hammer to drive out
the crankshaft.
REMOVING THE CRANKCASE
BEARING
1. Remove:• Bearing "1"
• Remove the bearing from the
crankcase by pressing its inner
race.
• Do not use the removed bearing.
CHECKING THE TIMING CHAIN
AND TIMING CHAIN GUIDE
1. Inspect:
•Timing chainCracks/stiff
→Replace the timing
chain and camshaft sprocket as a
set.
2. Inspect: • Timing chain guide
Wear/damage
→Replace.
CHECKING THE CRANKCASE
1. Inspect: • Contacting surface "a"Scratches
→Replace.
• Engine mounting boss "b", crank- case
Cracks/damage
→Replace. 2. Inspect:
• BearingRotate inner race with a finger.
Rough spot/seizure
→Replace.
3. Inspect: •Oil seal
Damage
→Replace.
CHECKING THE CRANKSHAFT
1. Measure: • Runout limit "a"
• Small end free play limit "b"
• Connecting rod big end side clearance "c"
• Crank width "d" Out of specification
→Replace.
Use the dial gauge and a thick-
ness gauge.
Crankcase separating
tool: YU-1135-A/90890-
01135
Dial gauge and stand:YU-3097/90890-01252
Standard it>
Runou
t limit: 0.03 mm (0.0012
in) 0.05
mm
(0.002
in)
Small
end
free
play: 0.4–1.0 mm
(0.016–0.039 in) 2.0 mm
(0.08 in)
Side
clear-
ance: 0.15–0.45 mm
(0.0059–0.0177 in) 0.50
mm
(0.02 in)
Crack
width: 55.95–56.00
mm (2.203– 2.205 in) —

4-60
CRANKCASE AND CRANKSHAFT
5. Install:• Dowel pin "1"
• O-ring "2"
• Right crankcaseTo left crankcase.
• Apply the lithium soap base grease on the O-ring.
• Fit the right crankcase onto the left
crankcase. Tap lightly on the case
with soft hammer.
• When installing the crankcase, the
connecting rod should be posi-
tioned at TDC (top dead center).
6. Tighten:• Hose guide "1"
• Clutch cable holder "2"
• Bolt (crankcase) "3"
Tighten the crankcase tightening
bolts in stage, using a crisscross pat-
tern.
7. Install:• Oil delivery pipe 2 "1"
• O-ring "2"
• Bolt (oil delivery pipe 2) "3"
Apply the lithium soap base grease
on the Orings.
8. Install:
• Timing chain "1"
• Timing chain guide (intake side) "2"
• Bolt (timing chain guide) "3"
9. Remove: •Sealant
Forced out on the cylinder mating
surface.
10. Apply:
• Engine oilTo the crank pin, bearing and oil
delivery hole.
11. Check: • Crankshaft and transmission op-eration.
Unsmooth operation
→Repair.
Bolt (crankcase):
12 Nm (1.2 m•kg, 8.7
ft•lb)
Bolt (oil delivery pipe 2): 10 Nm (1.0 m•kg, 7.2
ft•lb)
Bolt (timing chain guide):10 Nm (1.0 m•kg, 7.2
ft•lb)

4-62
TRANSMISSION, SHIFT CAM AND SHIFT FORK
REMOVING THE TRANSMISSION
1. Remove:• Main axle "1"
•Drive axle "2"
•Shift cam
• Shift fork 3
• Shift fork 2
• Shift fork 1
• Remove assembly with the collar "3" installed to the crankcase.
• Remove assembly carefully. Note
the position of each part. Pay partic-
ular attention to the location and di-
rection of shift forks.
• Remove the main axle, drive axle, shift cam and shift fork all together
by tapping lightly on the transmis-
sion drive axle with a soft hammer.
CHECKING THE GEARS
1. Inspect:• Matching dog "a"
• Gear teeth "b"
• Shift fork groove "c"Wear/damage
→Replace.
2. Inspect: • O-ring "1"
Damage
→Replace.
3. Check: • Gears movement
Unsmooth movement
→Repair or
replace.
CHECKING THE BEARING
1. Inspect: • Bearing "1"
Rotate inner race with a finger.
Rough spot/seizure
→Replace. CHECKING THE SHIFT FORK,
SHIFT CAM AND SEGMENT
1. Inspect:
• Shift fork "1"
Wear/damage/scratches
→Re-
place.
2. Inspect: • Shift cam "1"
• Segment "2"Wear/damage
→Replace.
3. Check: • Shift fork movement
Unsmooth operation
→Replace
shift fork.
For a malfunctioning shift fork, re-
place not only the shift fork itself but
the two gears each adjacent to the
shift fork.
INSTALLING THE TRANSMISSION
1. Install: • 5th pinion gear (24T) "1"
• 3rd pinion gear (20T) "2"
•Collar "3"
• 4th pinion gear (22T) "4"
• 2nd pinion gear (16T) "5"To main axle "6".
Apply the molybdenum disulfide oil
on the inner and end surface of the idler gear and on the inner surface of
the sliding gear, then install.
2. Install:
• 2nd wheel gear (28T) "1"
• 4th wheel gear (27T) "2"
• 3rd wheel gear (29T) "3"
• 5th wheel gear (25T) "4"
• 1st wheel gear (30T) "5"
• O-ring "6" To drive axle "7".
• Apply the molybdenum disulfide oil on the inner and end surface of the
idler gear and on the inner surface
of the sliding gear, then install.
• Apply the lithium soap base grease on the O-ring.
3. Install:
• Washer "1"
• Circlip "2"
• Be sure the circlip sharp-edged cor-ner "a" is positioned opposite side
to the washer and gear "b".
• Install the circlip with its ends "c" settled evenly on the spline crests.