2004 YAMAHA YZ250F engine

3 - 42
INSP
ADJ
STEERING HEAD INSPECTION AND ADJUSTMENT
2. Inspect:
Bearing free play
Exist play → Replace.
EC36U013
STEERING HEAD INSPECTION AND
ADJUSTMENT
1. Elevate the front wheel by placing a suit-
able stand under the engine.
2. Check:
Steering stem
Grasp the bottom of the forks and gen-
tly rock the fork assembly back and
forth.
Free play → Adjust steering head.
3. Check:
Steering smooth action
Turn the handlebar lock to lock.
Unsmooth action → Adjust steering ring
nut.
4. Adjust:
Steering ring nut
Steering ring nut adjustment steps:
Remove the number plate.
Remove the handlebar and handlebar
upper bracket.
Loosen the steering ring nut 1 using the
steering nut wrench 2.
Steering nut wrench:
YU-33975/90890-01403

3 - 45
INSP
ADJ
ELECTRICAL/SPARK PLUG INSPECTION
EC370000
ELECTRICAL
EC371001
SPARK PLUG INSPECTION
1. Remove:
Spark plug
2. Inspect:
Electrode 1
Wear/damage → Replace.
Insulator color 2
Normal condition is a medium to light
tan color.
Distinctly different color → Check the
engine condition.
NOTE:
When the engine runs for many hours at low
speeds, the spark plug insulator will become
sooty, even if the engine and carburetor are in
good operating condition.
3. Measure:
Plug gap a
Use a wire gauge or thickness gauge.
Out of specification → Regap.
4. Clean the plug with a spark plug cleaner
if necessary.
Spark plug gap:
0.7 ~ 0.8 mm (0.028 ~ 0.031 in)
5. Tighten:
Spark plug
NOTE:
Before installing a spark plug, clean the gas-
ket surface and plug surface.
Finger-tighten a the spark plug before
torquing to specification b.
T R..13 Nm (1.3 m · kg, 9.4 ft · lb)

3 - 46
INSP
ADJ
IGNITION TIMING CHECK
IGNITION TIMING CHECK
1. Remove:
Timing mark accessing screw 1
2. Attach:
Timing light
Inductive tachometer
To the ignition coil lead (orange lead
1).
Timing light:
YM-33277-A/90890-03141
1
3. Adjust:
Engine idling speed
Refer to “ENGINE IDLING SPEED
ADJUSTMENT” section.
4. Check:
Ignition timing
Visually check the stationary pointer a
is within the firing range b on the rotor.
Incorrect firing range → Check rotor
and pickup assembly.
5. Install:
Timing mark accessing screw

4 - 1
ENG

EC400000
ENGINE
SEAT, FUEL TANK AND SIDE COVERS
Extent of removal:
1
Seat removal
2
Fuel tank removal
3
Side covers removal
4
Number plate removal
Extent of removal Order Part name Q’ty Remarks
Preparation for removal
SEAT, FUEL TANK AND SIDE
COVERS REMOVAL
Turn the fuel cock to “OFF”.
Disconnect the fuel hose.
1 Seat 1
2 Air scoop (left and right) 2
3 Fitting band 1 Remove on fuel tank side.
4 Bolt (fuel tank) 2
5 Fuel tank 1
6 Left side cover 1
Refer to “REMOVAL POINTS”.
7 Right side cover 1
8 Number plate 1
13
4
2
3

SEAT, FUEL TANK AND SIDE COVERS
4

4 - 6
ENG

RADIATOR
EC456000
HANDLING NOTE
WARNING

Do not remove the radiator cap when the
engine and radiator are hot. Scalding hot
fluid and steam may be blown out under
pressure, which could cause serious
injury.
When the engine has cooled, open the radi-
ator cap by the following procedure:
Place a thick rag, like a towel, over the radi-
ator cap, slowly rotate the cap counter-
clockwise to the detent. This procedure
allows any residual pressure to escape.
When the hissing sound has stopped,
press down on the cap while turning coun-
terclockwise and remove it.
EC454000
INSPECTION
EC444100
Radiator
1. Inspect:

Radiator core
1

Obstruction
→
Blow out with com-
pressed air through rear of the radiator.
Bent fin → Repair/replace.
EC455000
ASSEMBLY AND INSTALLATION
Radiator
1. Install:
O-ring 1
Radiator pipe 2
Bolt (radiator pipe) 3
NOTE:
Apply the lithium soap base grease on the O-
ring.
2. Install:
Radiator breather hose 1
Radiator hose 4 (longer) 2
Radiator hose 2 (shorter) 3
To left radiator 4.
T R..10 Nm (1.0 m · kg, 7.2 ft · lb)

4 - 11
ENGCARBURETOR
HANDLING NOTE
CAUTION:
Do not loosen the screws (throttle position
sensor) 1 except when changing the throt-
tle position sensor due to failure because it
will cause a drop in engine performance.
1
REMOVAL POINTS
Pilot screw
1. Remove:
Pilot screw 1
NOTE:
To optimize the fuel flow at a small throttle
opening, each machine’s pilot screw has been
individually set at the factory. Before removing
the pilot screw, turn it in fully and count the
number of turns. Record this number as the
factory-set number of turns out.
INSPECTION
Carburetor
1. Inspect:
Carburetor body
Contamination → Clean.
NOTE:
Use a petroleum based solvent for cleaning.
Blow out all passages and jets with com-
pressed air.
Never use a wire.

4 - 26
ENG

CAMSHAFTS
Decompression system
1. Check:

Decompression system
Checking steps:

Check that the decompression mecha-
nism cam
1
moves smoothly.

Check that the decompression mechanism
cam lever pin
2
projects from the camshaft.
Timing chain tensioner
1. Check:

While pressing the tensioner rod lightly
with fingers, use a thin screwdriver
1
and wind the tensioner 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 assembly.
ASSEMBLY AND INSTALLATION
Camshaft
1. Install:

Exhaust camshaft
1


Intake camshaft
2

Installation steps:

Turn the crankshaft counterclockwise with
a wrench.
NOTE:


Apply the molybdenum disulfide oil on the
camshafts.

Apply the engine oil on the decompression
system.

Squeezing the decompression lever
allows the crankshaft to be turned easily.

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.

4 - 27
ENG

CAMSHAFTS

Fit the timing chain
3
onto both camshaft
sprockets and install the camshafts on the
cylinder head.
NOTE:

The camshafts should be installed onto the
cylinder head so that the punch mark
c
on
the exhaust camshaft and the punch mark
d
on the intake camshaft must align with
the cylinder head surface, as shown in the
illustration.
CAUTION:
Do not turn the crankshaft during the
camshaft installation. Damage or

improper valve timing will result.

Install the clips, camshaft caps
4
and
bolts (camshaft cap)
5
.
T R..

Bolt (camshaft cap):
10 Nm (1.0 m • kg, 7.2 ft • lb)
NOTE:


Before removing the clips, cover the cylin-
der head with a clean rag to prevent the
clips from into the cylinder head cavity.

Apply the engine oil on the thread and con-
tact surface of the bolts (camshaft cap).

Tighten the bolts (camshaft cap) in a criss-
cross pattern.
CAUTION:
The bolts (camshaft cap) must be tight-
ened evenly, or damage to the cylinder
head, camshaft caps, and camshaft will

result.
2. Install:

Timing chain tensioner
Installation steps:

While pressing the tensioner rod lightly
with fingers, use a thin screwdriver and
wind the tensioner rod up fully clockwise.