YAMAHA WR 250F 2008 Owners Manual

5-5
CARBURETOR
DISASSEMBLING THE CARBURETOROrder Part name Q'ty Remarks1 Carburetor breather hose 4
2 Valve lever housing cover 1
3 Screw (throttle shaft) 1
4 Throttle valve 1
5 Needle holder 1
6 Jet needle 1
7 Accelerator pump cover 1
8Spring 1
9 Diaphragm (accelerator pump) 1
10 Air cut valve cover 1
11 Spring (air cut valve) 1
12 Diaphragm (air cut valve) 1
13 Float chamber 1
14 Leak jet 1
15 Pilot screw 1 Refer to removal section.
16 Float pin 1
17 Float 1
18 Needle valve 1

5-6
CARBURETOR
19 Main jet1
20 Needle jet 1
21 Spacer 1
22 Pilot jet 1
23 Starter jet 1
24 Push rod 1 Pull the push rod.
25 Throttle shaft assembly 1
26 Push rod link lever assembly 1
27 Main air jet 1
28 Pilot air jet 1
29 Cold starter plunger 1
Order
Part name Q'tyRemarks

5-7
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" • Main air jet "7"
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. 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 ca rburetor 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.
Float height:
8.0 mm (0.31 in)

5-8
CARBURETOR
f. Recheck the float height.
CHECKING THE FLOAT
1. Inspect:• Float "1"Damage →Replace.
CHECKING THE STARTER
PLUNGER
1. Inspect: • Cold starter plunger "1"
• Hot starter plunger "2"Wear/damage →Replace.
CHECKING THE ACCELERATOR
PUMP
1. Inspect: • Diaphragm (accelerator pump) "1"
• Spring (accelerator pump) "2"
• Accelerator pump cover "3"
• O-ring "4"
• Push rod "5" Tears (diaphragm)/damage →Re-
place.
Dirt →Clean. 2. Inspect:
• Throttle shaft "1"
• Spring "2"
• Lever 1 "3"
• Spring 1 "4"
• Lever 2 "5"
• Spring 2 "6"Dirt →Clean.
CHECKING THE AIR CUT VALVE
1. Inspect: • Diaphragm (air cut valve) "1"
• Spring (air cut valve) "2"
• Air cut valve cover "3"
• O-ring "4"
Tears (diaphragm)/damage →Re-
place.
ASSEMBLING THE CARBURETOR
1. Install: • Cold starter plunger
2. Install:
•Pilot air jet "1"
• Main air jet "2"
3. Install: • Spring 1 "1"
• Lever 1 "2"
To lever 2 "3".
Make sure the spring 1 fits on the
stopper "a" of the lever 2.
4. Install:• Spring 2 "1"To lever 2 "2".
5. Install: • Push rod link lever assembly "1"
Make sure the stopper "a" of the
spring 2 fits into the recess "b" in the
carburetor.
6. Install:• Washer "1"
• Circlip "2"
7. Install: • Spring "1"To throttle shaft "2".
Install the bigger hook "a" of the
spring fits on the stopper "b" of the
throttle shaft pulley.
8. Install:• Throttle shaft assembly "1"
• Washer (metal) "2"
• Washer (resin) "3"
• Valve lever "4"

5-9
CARBURETOR
• Apply the fluorochemical grease on the bearings.
• Fit the projection "a" on the throttle shaft assembly into the slot "b" in
the throttle position sensor.
• Make sure the stopper "c" of the spring fits into the recess in the car-
buretor.
• Turn the throttle shaft assembly left while holding down the lever 1 "5"
and fit the throttle stop screw tip "d"
to the stopper "e" of the throttle
shaft assembly pulley.
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 (air cut valve) "1"
• Spring (air cut valve) "2"
• O-ring "3"
• Air cut valve cover "4"
• Holder (cylinder head breather hose) "5"
• Screw (air cut valve cover) "6"
15. 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.
16. Install:
• Jet needle "1"
•Collar "2"
• Spring "3"
• Needle holder "4"
• Throttle valve plate "5"To throttle valve "6".
Pilot screw (example): 1-1/2 turns out

5-10
CARBURETOR
17. Install:• Throttle valve assembly "1"
• Screw (throttle shaft) "2"
Install the valve lever rollers "3" into
the slits "a" of the throttle valve.
18. Install:• O-ring "1"
• Valve lever housing cover "2"
• Bolt (valve lever housing cover) "3"
19. 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"
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)

5-11
CARBURETOR
8. Install:• Throttle position sensor lead cou-
pler "1"
• Clamp "2" Refer to "CABLE ROUTING DIA-
GRAM" section in the CHAPTER
2.
9. Install: • Clamp "1"Refer to "CABLE ROUTING DIA-
GRAM" section in the CHAPTER
2.

5-12
AIR INDICTOIN SYSTEM
AIR INDICTOIN SYSTEM
REMOVING THE AIR INDUCTION SYSTEMOrder Part name Q'ty Remarks1 Bracket 1
2 Air cut-off valve assembly 1
3 Air induction hose (air cut-off valve - front of cyl-
inder head) 1
4 Air induction pipe 1
5 Gasket 1
6 Air induction hose (air cut-off valve - rear of cylin-
der head)
1
7 Air induction hose (air cut-off valve - air filter
case) 1
7 Nm (0.7 m kg, 5.1 ft lb)10 Nm (1.0 m kg, 7.2 ft lb)
4 Nm (0.4 m kg, 2.9 ft lb)

5-13
AIR INDICTOIN SYSTEM
CHECKING THE AIR INDUCTION
SYSTEM
1. Inspect:• Air induction hose
Crack/damage →Replace.
• Air induction pipe Crack/damage →Replace.
2. Check: • Operation of air cut valvePass air through the pipe and
check the air cut valve for opera-
tion.
Does not meet the following con-
dition →Replace the air cut valve
assembly.
• Blow in air to check for operation.
• When using vacuum, check by the use of the vacuum/pressure pump
gauge set "1".
When using vacuum on the pipe
"c", take care not to exceed the
specified value.
a. From air filter
b. To cylinder head (exhaust
port)
c. From cylinder head (intake port)
A. Check for induction from air filter.
B. Check for prevention of back-
flow into air filter.
C. Check for prevention of after- burn. (When throttle is closed
at sudden deceleration)
"a" to "b" Air passes.
"b" to "a" Air does not pass.
"a" to "b"
Air does not pass
when specified pres-
sure is on "c".
Vacuum/pressure pump
gauge set: YB-35956-A/90890-
06756
Vacuum specifying pres-
sure: 46.7–86.7 kPa (350–650
mmHg, 13.8–25.6 inHg)

5-14
CAMSHAFTS
CAMSHAFTS
REMOVING THE CYLINDER HEAD COVEROrder Part name Q'ty Remarks Seat and fuel tank Refer to "REMOVING THE SEAT, FUEL
TANK AND SIDE COVERS" section in the
CHAPTER 3.
Air cut-off valve assembly Refer to "AIR INDUCTION SYSTEM" 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