air filter YAMAHA YZ250F 2011 User Guide

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 body
Contamination→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 movement
Stick→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.
Standard clip position:
No.4 Groove
Float height:
8.0 mm (0.31 in)

4-13
CARBURETOR
17. Install:
• O-ring "1"
• Valve lever housing cover "2"
• Bolt (valve lever housing cover)
"3"
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 play
Refer 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)

5-36
REAR SHOCK ABSORBER
REAR SHOCK ABSORBER
REMOVING THE REAR SHOCK ABSORBER
Order 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.
2Rear 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 Lower spring guide 1
9 Upper spring guide 1
10 Spring (rear shock absorber) 1
11 Bearing 2 Refer to removal section.
B2
9
8 10
11
11

5-39
REAR SHOCK ABSORBER
6. Install:
• Rear frame "1"
• Bolt [rear frame (upper)] "2"
• Bolt [rear frame (lower)] "3"
7. Tighten:
• Bolt (air filter joint) "1"Bolt [rear frame (upper)]:
32 Nm (3.2 m•kg, 23
ft•lb)
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 SYMPTOMFor USA and CDN
Diameter of straight portion
RichLean
NPQRS TU
Rich 1 richer NDJN-5 NDJP-5 NDJQ-5 NDJR-5 NDJS-5 NDJT-5 NDJU-5
0.5 richer NCVN-4 NCVP-4 NCVQ-4 NCVR-4 NCVS-4 NCVT-4 NCVU-4
STD NDJN-4 NDJP-4 NDJQ-4 NDJR-4 NDJS-4 NDJT-4 NDJU-4
0.5 leaner NCVN-3 NCVP-3 NCVQ-3 NCVR-3 NCVS-3 NCVT-3 NCVU-3
Lean 1 leaner NDJN-3 NDJP-3 NDJQ-3 NDJR-3 NDJS-3 NDJT-3 NDJU-3
Except for USA and CDN
Diameter of straight portion
RichLean
NPQRS TU
Rich 1 richer NFLN-7 NFLP-7 NFLQ-7 NHKR-5 NFLS-7 NFLT-7 NFLU-7
0.5 richer NFPN-6 NFPP-6 NFPQ-6 NFPR-6 NFPS-6 NFPT-6 NFPU-6
STD NFLN-6 NFLP-6 NFLQ-6 NHKR-4 NFLS-6 NFLT-6 NFLU-6
0.5 leaner NFPN-5 NFPP-5 NFPQ-5 NFPR-5 NFPS-5 NFPT-5 NFPU-5
Lean 1 leaner NFLN-5 NFLP-5 NFLQ-5 NHKR-3 NFLS-5 NFLT-5 NFLU-5
Symptom Setting Checking
At full throttle
Hard breathing
Shearing noise
Whitish spark plug
↓
Lean mixtureIncrease 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 mixtureDecrease 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 speedLower jet needle clip position. (1 groove
down)
1/4–1/2 throttle
Slow speed pick-up
Poor accelerationRaise jet needle clip position. (1 groove up)
Jet needleClip
Groove 7 Groove 6
Groove 5
Groove 4
Groove 3
Groove 2
Groove 1
Leaner
(Standard)
Richer

7-5
ENGINE
This should be taken simply for an example. It is necessary to set the carburetor while checking the operating conditions
of the engine.
Closed to 1/4 throttle
Hard breathing
Speed downUse jet needle with a smaller diameter. Slow-speed-circuit passage
Clogged→Clean.
Overflow from carburetor
Closed to 1/4 throttle
Poor accelerationUse jet needle with a larger diameter.
Raise jet needle clip position. (1 groove up)
Poor response in the low to in-
termediate speedsRaise jet needle clip position.
If this has no effect, lower the jet needle clip
position.
Poor response when throttle is
opened quicklyCheck 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. Symptom Setting Checking