2008 YAMAHA WR 250F Owners Manual

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

5-15
CAMSHAFTS
REMOVING THE CAMSHAFTSOrder Part name Q'ty Remarks1 Timing mark accessing screw 1 Refer to removal section.
2 Crankshaft end accessing screw 1 Refer to removal section.
3 Timing chain tensioner cap bolt 1 Refer to removal section.
4 Timing chain tensioner 1 Refer to removal section.
5 Camshaft cap 2 Refer to removal section.
6 Clip 2 Refer to removal section.
7 Exhaust camshaft 1 Refe r to removal section.
8 Intake camshaft 1 Refer to removal section.

5-16
CAMSHAFTS
REMOVING THE CAMSHAFT
1. Remove:• Timing mark accessing screw "1"
• Crankshaft end accessing screw
"2"
2. Align: • T.D.C. markWith align mark.
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 t hat 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.
3. Remove: • Timing chain tensioner cap bolt
"1"
• Timing chain tensioner "2"
• Gasket 4. Remove:
• Bolt (camshaft cap) "1"
• Camshaft cap "2"
• Clip
Remove the bolts (camshaft cap) in a
crisscross pattern, working from the
outside in.
The bolts (camshaft cap) must be
removed evenly to prevent dam-
age to the cylinder head, cam-
shafts or camshaft caps.
5. Remove:• Exhaust camshaft "1"
• Intake camshaft "2"
Attach a wire "3" to the timing chain to
prevent it from falling into the crank-
case.
CHECKING THE CAMSHAFT
1. Inspect:• Cam lobe
Pitting/scratches/blue discolora-
tion →Replace.
2. Measure:
• Cam lobe length "a" and "b"Out of specification →Replace. 3. Measure:
• Runout (camshaft)Out of specification →Replace.
4. Measure: • Camshaft-to-cap clearance
Out of specification →Measure
camshaft outside diameter.
Cam lobes length: Intake "a":29.65–29.75 mm
(1.1673–1.1713 in)
: 29.55 mm (1.1634 in)
Intake "b": 22.45–22.55 mm
(0.8839–0.8878 in)
: 22.35 mm (0.8799 in)
Exhaust "a":
30.399–30.499 mm
(1.1968–1.2007 in)
:
30.299 mm (1.1929 in)
Exhaust "b": 22.45–22.55 mm
(0.8839–0.8878 in)
: 22.35 mm (0.8799 in)
Runout (camshaft): Less than 0.03 mm
(0.0012 in)