air filter YAMAHA YZ250F 2008 User Guide

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)

5-37
REAR SHOCK ABSORBER
REAR SHOCK ABSORBER
REMOVING THE REAR SHOCK ABSORBEROrder Part name Q'ty Remarks Hold the machine by placing the suitable stand
under the engine. Refer to "HANDLING NOTE".
Seat Refer to "SEAT, FUEL TANK AND SIDE
COVERS" section in the CHAPTER 4.
Silencer Refer to "EXHAUST PIPE AND SILENCER"
section in the CHAPTER 4.
1 Clamp (air filter joint) 1 Only loosening.
2 Rear frame 1
3 Bolt (rear shock absorber-relay arm) 1 Hold the swingarm.
4 Bolt (rear shock absorber-frame) 1
5 Rear shock absorber 1
6 Locknut 1 Only loosening.
7 Adjuster 1 Only loosening.
8 Spring seat 2
9 Lower spring guide 1
10 Upper spring guide 1
11 Spring (rear shock absorber) 1

5-41
REAR SHOCK ABSORBER
• Bolt [rear frame (lower)] "3"
7. Tighten: • Bolt (air filter joint) "1" Bolt [rear frame (lower)]:
32 Nm (3.2 m•kg, 23
ft•lb)
Bolt (air filter joint): 3 Nm (0.3 m•kg, 2.2
ft•lb)

7-4
ENGINE
SPECIFICATIONS OF JET NEEDLE
EXAMPLES OF CARBURETOR SETTING DEPENDING ON SYMPTOMDiameter of straight portion
Rich Lean
NPQRS TU
Rich 1 richer NFPN-6 NFPP-6 NFPQ-6 NFPR-6 NFPS-6 NFPT-6 NFPU-6 0.5 richer NFLN-6 NFLP-6 NFLQ-6 NFLR-6 NFLS-6 NFLT-6 NFLU-6STD NFPN-5 NFPP-5 NFPQ-5 NFPR-5 NFPS-5 NFPT-5 NFPU-5
0.5 leaner NFLN-5 NFLP-5 NFLQ-5 NFLR-5 NFLS-5 NFLT-5 NFLU-5
Lean 1 leaner NFPN-4 NFPP-4 NFPQ-4 NFPR-4 NFPS-4 NFPT-4 NFPU-4
Symptom Setting Checking
At full throttle Hard breathing
Shearing noise
Whitish spark plug
↓
Lean mixture Increase main jet calibration no. (Gradual-
ly)
Discoloration of spark plug
→If tan color, it
is in good condition.
If cannot be corrected: Clogged float valve seat
Clogged fuel hose
Clogged fuel cock
Check that the accelerator pump operates
smoothly.
At full throttle Speed pick-up stops
Slow speed pick-up
Slow response
Sooty spark plug
↓
Rich mixture Decrease main jet calibration no. (Gradual-
ly)
Discoloration of spark plug
→If tan color, it
is in good condition.
If cannot be corrected:
Clogged air filter
Fuel overflow from carburetor
Lean mixture Lower jet needle clip position. (1 groove
down)
The clip position is the jet needle groove on
which the clip is installed.
The positions are numbered from the top.
Check that the accelerator pump operates
smoothly. (except for rich mixture symp-
tom).
Rich mixture
Raise jet needle clip position. (1 groove up)
1/4–3/4 throttle Hard breathing
Lack of speed Lower jet needle clip position. (1 groove
down)
1/4–1/2 throttle Slow speed pick-up
Poor acceleration Raise jet needle clip position. (1 groove up)
Closed to 1/4 throttle Hard breathing
Speed down Use jet needle with a smaller diameter. Slow-speed-circuit passage
Clogged
→Clean.
Overflow from carburetor
Closed to 1/4 throttle Poor acceleration Use jet needle with a larger diameter.
Raise jet needle clip position. (1 groove up)
Poor response in the low to in-
termediate speeds Raise jet needle clip position.
If this has no effect, lower the jet needle clip
position.
Poor response when throttle is
opened quickly Check overall settings.
Use main jet with a lower calibration no.
Raise jet needle clip position. (1 groove up)
If these have no effect, use a main jet with
a higher calibration no. and lower the jet
needle clip position. Check air filter for fouling.
Check that the accelerator pump operates
smoothly.
Jet needleClip
Groove 7 Groove 6
Groove 5
Groove 4
Groove 3
Groove 2
Groove 1
Leaner
(Standard) Richer