valve YAMAHA YZ250F 2005 Service Manual

5 - 28
CHASFRONT FORK
EC556000
HANDLING NOTE
NOTE:
The front fork requires careful attention. So it is
recommended that the front fork be maintained
at the dealers.
CAUTION:
To prevent an accidental explosion of air,
the following instructions should be
observed:
The front fork with a built-in piston rod
has a very sophisticated internal con-
struction and is particularly sensitive to
foreign material.
Use enough care not to allow any foreign
material to come in when the oil is
replaced or when the front fork is disas-
sembled and reassembled.
Before removing the base valves or front
forks, be sure to extract the air from the
air chamber completely.
EC553000
REMOVAL POINTS
Damper assembly
1. Loosen:
Damper assembly 1
NOTE:
Before removing the front fork from the
machine, loosen the damper assembly with
the cap bolt ring wrench 2.
Cap bolt ring wrench:
YM-01501/90890-01501

5 - 30
CHAS
Base valve
1. Remove:
Base valve 1
From damper assembly 2.
NOTE:
Hold the damper assembly with the cap bolt
ring wrench 3 and use the cap bolt wrench 4
to remove the base valve.
Cap bolt wrench:
YM-01500/90890-01500
Cap bolt ring wrench:
YM-01501/90890-01501
EC554000
INSPECTION
Damper assembly
1. Inspect:
Damper assembly 1
Bend/damage → Replace.
O-ring 2
Wear/damage → Replace.
CAUTION:
The front fork with a built-in piston rod has
a very sophisticated internal construction
and is particularly sensitive to foreign
material.
Use enough care not to allow any foreign
material to come in when the oil is replaced
or when the front fork is disassembled and
reassembled.
FRONT FORK

5 - 31
CHAS
Base valve
1. Inspect:
Base valve 1
Wear/damage → Replace.
Contamination → Clean.
O-ring 2
Wear/damage → Replace.
Bushing 3
Wear/damage → Replace base valve.
Spring 4
Damage/fatigue → Replace base valve.
Air bleed screw 5
Wear/damage → Replace base valve.
EC554400
Fork spring
1. Measure:
Fork spring free length a
Out of specification → Replace.
EC554502
Inner tube
1. Inspect:
Inner tube surface a
Score marks → Repair or replace.
Use #1,000 grit wet sandpaper.
Damaged oil lock piece → Replace.
Inner tube bends
Out of specification → Replace.
Use the dial gauge 1.
NOTE:
The bending value is shown by one half of the
dial gauge reading.
WARNING
Do not attempt to straighten a bent inner
tube as this may dangerously weaken the
tube.
Fork spring free length:
465 mm (18.3 in)
: 460 mm (18.1 in)
Inner tube bending limit:
0.2 mm (0.008 in)
FRONT FORK

5 - 33
CHASFRONT FORK
4. After filling, pump the damper assembly 1
slowly up and down (about 200 mm (7.9 in)
stroke) several times to bleed the damper
assembly of air.
NOTE:
Be careful not to excessive full stroke. A stroke
of 200 mm (7.9 in) or more will cause air to
enter. In this case, repeat the steps 2 to 4.
5. Inspect:
Oil amount
While keeping the damper assembly fully
stretched, check that the oil is above the
step a on the damper assembly.
Below the step → Replenish fork oil
above the step.
6. Tighten:
Locknut 1
NOTE:
Fully finger tighten the locknut onto the
damper assembly.
7. Loosen:
Compression damping adjuster 1
NOTE:
Loosen the compression damping adjuster
finger tight.
Record the set position of the adjuster (the
amount of turning out the fully turned in posi-
tion).
8. Install:
Base valve 1
To damper assembly 2.
NOTE:
First bring the damper rod pressure to a maxi-
mum. Then install the base valve while releas-
ing the damper rod pressure.

5 - 34
CHASFRONT FORK
9. Tighten:
Base valve 1
NOTE:
Hold the damper assembly with the cap bolt
ring wrench 2 and use the cap bolt wrench 3
to tighten the base valve with specified torque.
Cap bolt wrench:
YM-01500/90890-01500
Cap bolt ring wrench:
YM-01501/90890-01501
T R..29 Nm (2.9 m · kg, 21 ft · lb)
10. After filling, pump the damper assembly 1
slowly up and down more than 10 times to
distribute the fork oil.
11. While protecting the damper assembly 1
with a rag and compressing fully, allow
excessive oil to overflow on the base valve
side.
CAUTION:
Take care not to damage the damper
assembly.
12. Allow the overflowing oil to escape at the
hole a in the damper assembly.

5 - 37
CHASFRONT FORK
21. Check:
Inner tube smooth movement
Tightness/binding/rough spots → Repeat
the steps 14 to 20.
22. Measure:
Distance a
Out of specification → Turn into the lock-
nut.
Distance
a:
19 mm (0.75 in) or more
Between damper assembly
1
bottom and locknut
2 bottom.
23. Install:
Spacer (metal) 1
Fork spring 2
Spacer (resin) 3
To damper assembly 4.
NOTE:
Install the spacer with the plate a facing the
fork spring.
24. Install:
Damper assembly 1
To inner tube 2.
CAUTION:
To install the damper assembly into the
inner tube, hold the inner tube aslant. If the
inner tube is held vertically, the damper
assembly may fall into it, damaging the
valve inside.
25. Loosen:
Rebound damping adjuster 1
NOTE:
Loosen the rebound damping adjuster finger
tight.
Record the set position of the adjuster (the
amount of turning out the fully turned in posi-
tion).

5 - 63
CHAS
HANDLING NOTE
WARNING
This rear shock absorber is provided with a
separate type tank filled with high-pressure
nitrogen gas. To prevent the danger of
explosion, read and understand the follow-
ing information before handling the shock
absorber.
The manufacturer can not be held respon-
sible for property damage or personal
injury that may result from improper han-
dling.
1. Never tamper or attempt to disassemble
the cylinder or the tank.
2. Never throw the rear shock absorber
into an open flame or other high heat.
The rear shock absorber may explode as
a result of nitrogen gas expansion and/
or damage to the hose.
3. Be careful not to damage any part of the
gas tank. A damaged gas tank will impair
the damping performance or cause a
malfunction.
4. Take care not to scratch the contact sur-
face of the piston rod with the cylinder;
or oil could leak out.
5. Never attempt to remove the plug at the
bottom of the nitrogen gas tank. It is
very dangerous to remove the plug.
6. When scrapping the rear shock
absorber, follow the instructions on dis-
posal.
NOTES ON DISPOSAL (YAMAHA DEALERS
ONLY)
Before disposing the rear shock absorber, be
sure to extract the nitrogen gas from valve 1.
Wear eye protection to prevent eye damage
from escaping gas and/or metal chips.
WARNING
To dispose of a damaged or worn-out rear
shock absorber, take the unit to your
Yamaha dealer for this disposal procedure.
REAR SHOCK ABSORBER

–+ELEC
6 - 2
MAP-CONTROLLED CDI UNIT
MAP-CONTROLLED CDI UNIT
A map-controlled, CDI ignition system is used in the YZ250F.
The microcomputer in the CDI unit detects the engine speed and throttle position, thus determining
the optimum ignition timing through the entire operating range. In this way, quick throttle response
can be achieved according to various riding conditions.
Throttle position sensorCDI unitIgnition coil
Pickup coilCDI magneto rotor

Function of Component
Component Function
Throttle position sensor Detects throttle valve opening and inputs it into the computer in the
CDI unit as a throttle opening signal.
Pickup coil Detects signal rotor revolutions and inputs them into the computer in
the CDI unit as engine revolution signals.
CDI unit The signals of the throttle position sensor and pickup coil sensor are
analyzed by the computer in the CDI unit, which then adjusts ignition
timing for the operation requirements.

Principal of 3-Dimensional Control
Conventionally, ignition timing was controlled only
by engine revolutions (2-dimensional control).
However, ignition timing needs advancement also
by engine load. Thus, accurate ignition timing can
be determined by adding throttle opening to deter-
mine ignition timing (3-dimensional control).
3-D Image Map of Ignition Timing
(different from actual characteristics)
Ignition timing
Thr
ot
tl
e
ope
ni
ngRevo
l
ut
io
ns

7 - 3
TUNSETTING
Effects of the setting parts on the throttle
valve opening
ÈClosed
ÉFully open
1Pilot screw/pilot jet
2Throttle valve cutaway
3Jet needle
4Main jet
1/2 3/4 1/4 1/8
1
2
3
4
ÈÉ
Main system
The FLATCR carburetor has a primary main
jet. This type of main jet is perfect for racing
machines since it supplies an even flow of fuel,
even at full load. Use the main jet and the jet
needle to set the carburetor.
Pilot system
The FLATCR carburetor is manufactured with
a pilot screw. The pilot screw adjustment
ranges from fully closed throttle to 1/4 open
throttle.

7 - 10
TUN
Examples of carburetor setting depending on symptom
This should be taken simply for an example. It is necessary to set the carburetor while checking
the operating conditions of the engine.
Symptom Setting Checking
At full throttle
Hard breathing
Shearing noise
Whitish spark plug
Lean mixtureIncrease main jet calibration no.
(Gradually)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 oper-
ates smoothly.
At full throttle
Speed pick-up stops
Slow speed pick-up
Slow response
Sooty spark plug
Rich mixtureDecrease main jet calibration no.
(Gradually)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 oper-
ates smoothly. (except for rich mixture
symptom). 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)
Closed to 1/4 throttle
Hard breathing
Speed downUse jet needle with a smaller diame-
ter.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
intermediate speedsRaise jet needle clip position.
If this has no effect, lower the jet nee-
dle 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 oper-
ates smoothly.
Jet needleClip
Groove 7 Groove 6
Groove 5
Groove 4
Groove 3
Groove 2
Groove 1
Leaner
↑
(Standard)
↓
Richer
*
SETTING