2008 YAMAHA YZ250F fuel

4-7
CARBURETOR
CARBURETOR
REMOVING THE CARBURETOROrder Part name Q'ty Remarks Seat and fuel tank Refer to "SEAT, FUEL TANK AND SIDE
COVERS" section.
Rear shock absorber Refer to "REAR SHOCK ABSORBER" sec-
tion in the CHAPTER 5.
1Clamp 1
2 Throttle position sensor lead coupler 1
3 Throttle cable cover 1
4 Throttle cable 2
5 Clamp (air filter joint) 1 Loosen the screw (air filter joint).
6 Clamp (carburetor joint) 1 Loosen the screws (carburetor joint).
7 Hot starter plunger 1
8 Carburetor assembly 1
9 Carburetor joint 1
4 Nm (0.4 m kg, 2.9 ft lb)
4 Nm (0.4 m kg, 2.9 ft lb)
2 Nm (0.2 m kg, 1.4 ft lb)
3 Nm (0.3 m kg, 2.2 ft lb)
3 Nm (0.3 m kg, 2.2 ft lb)
11 Nm (1.1 m kg, 8.0 ft lb)
10 Nm (1.0 m kg, 7.2 ft lb)

4-10
CARBURETOR
HANDLING NOTE
Do not loosen the screw (throttle
position sensor) "1" except when
changing the throttle position sen-
sor due to failure because it will
cause a drop in engine perfor-
mance.
REMOVING THE PILOT SCREW
1. Remove:• Pilot screw "1"
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.
CHECKING THE CARBURETOR
1. Inspect:• Carburetor bodyContamination
→Clean.
• Use a petroleum based solvent for
cleaning. Blow out all passages and
jets with compressed air.
• Never use a wire.
2. Inspect:
•Main jet "1"
• Pilot jet "2"
• Needle jet "3"
• Starter jet "4"
• Pilot air jet "5"
• Leak jet "6" Damage
→Replace.
Contamination
→Clean.
• Use a petroleum based solvent for cleaning. Blow out all passages and
jets with compressed air.
• Never use a wire.
CHECKING THE NEEDLE VALVE
1. Inspect: • Needle valve "1"
• Valve seat "2"Grooved wear "a"
→Replace.
Dust "b"
→Clean.
•Filter "c" Clogged
→Clean.
CHECKING THE THROTTLE
VALVE
1. Check: • Free movementStick
→Repair or replace.
Insert the throttle valve "1" into the
carburetor body, and check for free
movement.
CHECKING THE JET NEEDLE
1. Inspect:• Jet needle "1"Bends/wear
→Replace.
• Clip groove
Free play exists/wear
→Replace.
• Clip position MEASURING AND ADJUSTING
THE FLOAT HEIGHT
1. Measure:
• Float height "a"
Out of specification
→Adjust.
Measurement and adjustment
steps:
a. Hold the carburetor in an upside down position.
• Slowly tilt the carburetor in the op-posite direction, then take the mea-
surement when the needle valve
aligns with the float arm.
• If the carburetor is level, the weight
of the float will push in the needle
valve, resulting in an incorrect mea-
surement.
b. Measure the distance between the mating surface of the float
chamber and top of the float using
a vernier calipers.
The float arm should be resting on the
needle valve, but not compressing
the needle valve.
c. If the float height is not within specification, inspect the valve
seat and needle valve.
d. If either is worn, replace them
both.
e. If both are fine, adjust the float height by bending the float tab "b"
on the float.
Standard clip position:No.5 Groove
Float height:
8.0 mm (0.31 in)

4-14
CAMSHAFTS
CAMSHAFTS
REMOVING THE CYLINDER HEAD COVEROrder Part name Q'ty Remarks Seat and fuel tank Refer to "SEAT, FUEL TANK AND SIDE
COVERS" section.
1 Spark plug 1
2 Cylinder head breather hose 1
3 Bolt (cylinder head cover) 2
4 Cylinder head cover 1
5 Cylinder head cover gasket 1
6 Timing chain guide (top side) 1

4-19
CYLINDER HEAD
CYLINDER HEAD
REMOVING THE CYLINDER HEADOrder Part name Q'ty Remarks Seat and fuel tank Refer to "SEAT, FUEL TANK AND SIDE
COVERS" section.
Exhaust pipe and silencer Refer to "EXHAUST PIPE AND SILENCER"
section.
Radiator hose 1 Disconnect at cylinder head side.
Carburetor Refer to "CARBURETOR" section.
Camshaft Refer to "CAMSHAFTS" section.
Upper engine bracket Refer to "ENGINE REMOVAL" section.
1 Radiator pipe 1
2 Oil delivery pipe 1
3Nut 2
4 Bolt [L = 135 mm (5.31 in)] 2
5 Bolt [L = 145 mm (5.71 in)] 2
6 Cylinder head 1
7 Timing chain guard (exhaust side) 1

4-49
CDI MAGNETO
CDI MAGNETO
REMOVING THE CDI MAGNETOOrder Part name Q'ty Remarks Seat and fuel tank Refer to "SEAT, FUEL TANK AND SIDE
COVERS" section.
Disconnect the CDI magneto lead.
Shift pedal Refer to "KICK SHAFT AND SHIFT SHAFT"
section.
1 Left crankcase cover 1
2 Nut (rotor) 1 Refer to removal section.
3 Rotor 1 Refer to removal section.
4 Stator 1
5 Woodruff key 1

4-51
ENGINE REMOVAL
ENGINE REMOVAL
REMOVING THE ENGINEOrder Part name Q'ty Remarks Hold the machine by placing the suitable stand
under the frame. Refer to "HANDLING NOTE".
Seat and fuel tank Refer to "SEAT, FUEL TANK AND SIDE
COVERS" section.
Carburetor Refer to "CARBURETOR" section.
Exhaust pipe and silencer Refer to "EXHAUST PIPE AND SILENCER"
section.
Clutch cable Disconnect at the engine side.
Radiator Refer to "RADIATOR" section.
Shift pedal Refer to "KICK SHAFT AND SHIFT SHAFT"
section.
Cylinder head breather hose Refer to "CAMSHAFTS" section.
Drain the engine oil. Refer to "CHANGING THE ENGINE OIL"
section in the CHAPTER 3.
Oil hose and oil tank breather hose Refer to "OIL PUMP" section.
Ignition coil
Disconnect the CDI magneto lead.
Right engine guard

5-16
FRONT FORK
FRONT FORK
REMOVING THE FRONT FORKOrder Part name Q'ty Remarks Hold the machine by placing the suitable stand
under the engine. Refer to "HANDLING NOTE".
Front wheel Refer to "FRONT WHEEL AND REAR
WHEEL" section.
Front brake caliper Refer to "FRONT BRAKE AND REAR
BRAKE" section.
Number plate Refer to "SEAT, FUEL TANK AND SIDE
COVERS" section in the CHAPTER 4.
1Protector 1
2 Pinch bolt (upper bracket) 2 Only loosening.
3 Damper assembly 1Loosen when disassembling the front fork.
Refer to removal section.
4 Pinch bolt (lower bracket) 2 Only loosening.
5 Front fork 1
21 Nm (2.1 m kg, 15 ft lb)
21 Nm (2.1 m kg, 15 ft lb)
5 Nm (0.5 m kg, 3.6 ft lb)
5 Nm (0.5 m kg, 3.6 ft lb)
7 Nm (0.7 m kg, 5.1 ft lb)

5-28
STEERING
STEERING
REMOVING THE STEERINGOrder Part name Q'ty Remarks TIGHTENING STEPS:
ŒTighten ring nut. 38 Nm (3.8 m•kg, 27 ft•lb)
ΠLoosen it one turn.
Œ Retighten it. 7 Nm (0.7 m•kg, 5.1 ft•lb)
Hold the machine by placing the suitable stand
under the engine. Refer to "HANDLING NOTE".
Number plate Refer to "SEAT, FUEL TANK AND SIDE
COVERS" section in the CHAPTER 4.
Handlebar Refer to "HANDLEBAR" section.
Front fender
1 Steering stem nut 1
2 Front fork 2 Refer to "FRONT FORK" section.
3 Upper bracket 1
4 Steering ring nut 1 Refer to removal section.
5 Lower bracket 1
6 Bearing race cover 1
7 Upper bearing 1
8 Lower bearing 1 Refer to removal section.