2008 YAMAHA WR 250F engine

3-27
ELECTRICAL
ELECTRICAL
CHECKING THE SPARK PLUG
1. Remove:• Spark plug
2. Inspect:
• Electrode "1"Wear/damage →Replace.
• Insulator color "2"
Normal condition is a medium to
light tan color.
Distinctly different color →Check
the engine condition.
When the engine runs for many hours
at low speeds, the spark plug insula-
tor will become sooty, even if the en-
gine and carburetor are in good
operating condition.
3. Measure: • Plug gap "a"Use a wire gauge or thickness
gauge.
Out of specification →Regap.
4. Clean the plug with a spark plug cleaner if necessary.
5. Tighten: • Spark plug
• Before installing a spark plug, clean the gasket surface and plug sur-
face.
• Finger-tighten "a" the spark plug before torquing to specification "b".
CHECKING THE IGNITION TIMING
1. Remove:• Timing mark accessing screw "1"
2. Attach: • Timing light
• Inductive tachometerTo the ignition coil lead (orange
lead"1").
3. Adjust: • Engine idling speedRefer to "ADJUSTING THE EN-
GINE IDLING SPEED" section.
4. Check: • Ignition timingVisually check the stationary
pointer "a" is within the firing
range "b" on the rotor.
Incorrect firing range →Check ro-
tor and pickup assembly.
5. Install: • Timing mark accessing screw
CHECKING AND CHARGING THE
BATTERY
Batteries generate explosive hy-
drogen gas and contain electrolyte
which is made of poisonous and
highly caustic sulfuric acid. There-
fore, always follow these preven-
tive measures:
• Wear protective eye gear when handling or working near batter-
ies. • Charge batteries
in a well-venti-
lated area.
• Keep batteries away from fire, sparks or open flames (e.g.,
welding equipment, lighted ciga-
rettes).
• DO NOT SMOKE when charging
or handling batteries.
• KEEP BATTERIES AND ELEC- TROLYTE OUT OF REACH OF
CHILDREN.
• Avoid bodily contact with elec- trolyte as it can cause severe
burns or permanent eye injury.
FIRST AID IN CASE OF BODILY
CONTACT:
EXTERNAL
• Skin — Wash with water.
• Eyes — Flush with water for 15
minutes and get immediate med-
ical attention.
INTERNAL
• Drink large quantities of water or milk followed with milk of magne-
sia, beaten egg or vegetable oil.
Get immediate medical attention.
Charging time, charging amperage
and charging voltage for an MF
battery are different from those of
conventional batteries. The MF
battery should be charged as ex-
plained in the charging method il-
lustrations. If the battery is
overcharged, the electrolyte level
will drop considerably. Therefore,
take special care when charging
the battery.
Since MF batteries are sealed, it is
not possible to check the charge state
of the battery by measuring the spe-
cific gravity of the electrolyte. There-
fore, the charge of the battery has to
be checked by measuring the voltage
at the battery terminals.
1. Remove:• Seat
2. Disconnect:
• Battery leads(from the battery terminals)
First, disconnect the negative bat-
tery lead "1", and then the positive
battery lead "2".
Spark plug gap: 0.7–0.8 mm (0.028–
0.031 in)
Spark plug: 13 Nm (1.3 m•kg, 9.4
ft•lb)
Timing light:
YM-33277-A/90890-
03141

4-1
ENGINE
TUNING
ENGINE
CARBURETOR SETTING
• The air/fuel mixture will vary de-pending on atmospheric conditions.
Therefore, it is necessary to take
into consideration the air pressure,
ambient temperature, humidity,
etc., when adjusting the carburetor.
• Perform a test run to check for prop- er engine performance (e.g., throt-
tle response) and spark plug(-s)
discoloration or fouling. Use these
readings to determine the best pos-
sible carburetor setting.
It is recommended to keep a record of
all carburetor settings and external
conditions (e.g., atmospheric condi-
tions, track/surface conditions, lap
times) to make future carburetor set-
ting easier.
• The carburetor is a part of the fuel line. Therefore, be sure to in-
stall it in a wellventilated area,
away from flammable objects
and any sources of fire.
• Never look into the carburetor in- take. Flames may shoot out from
the pipe if the engine backfires
while it is being started. Gasoline
may be discharged from the ac-
celerator pump nozzle when the
carburetor has been removed.
• The carburetor is extremely sen-sitive to foreign matter (dirt,
sand, water, etc.). During instal-
lation, do not al low foreign mat-
ter to get into the carburetor.
• Always handle the carburetor and its components carefully.
Even slight scratches, bends or
damage to carburetor parts may
prevent the carburetor from
functioning correctly. Carefully
perform all servicing with the ap-
propriate tools and without ap-
plying excessive force.
• When the engine is stopped or when riding at no load, do not
open and close the throttle un-
necessarily. Otherwise, too
much fuel may be discharged,
starting may become difficult or
the engine may not run well. • After installing the carburetor,
check that the throttle operates
correctly and opens and closes
smoothly.
ATMOSPHERIC CONDITIONS AND
CARBURETOR SETTINGS
The air density (i.e., concentration of
oxygen in the air) determines the rich-
ness or leanness of the air/fuel mix-
ture.
• Higher temperature expands the air with its resultant reduced density.
• Higher humidity reduces the amount of oxygen in the air by so
much of the water vapor in the
same air.
• Lower atmospheric pressure (at a
high altitude) reduces the density of
the air.
EFFECT OF SETTING PARTS IN
RELATION TO THROTTLE VALVE
OPENING
A. Closed
B. Fully open
1. Pilot jet
2. Throttle valve cutaway
3. Jet needle
4. Main jet CONSTRUCTION OF
CARBURETOR AND SETTING
PARTS
The FLATCR carburetor has a prima-
ry 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.
1. Jet needle
2. Pilot air jet
3. Needle jet
4. Main jet
5. Pilot jet
ADJUSTING THE MAIN JET
The richness of the air-fuel mixture at
full throttle can be set by changing the
main jet "1".
If the air-fuel mixture is too rich or too
lean, the engine power will drop, re-
sulting in poor acceleration.
Effects of changing the main jet
(reference)
A. Idle
B. Fully open
1. #180
2. #160
3. #170
Air
tem p. Hu-
midi- ty Air
pres- sure
(alti-
tude) Mix-
ture Set-
ting
High High Low
(high) Rich-
er Lean-
er
Low Low High
(low) Lean-
er Rich-
er
Standard main jet #170
* #160
* For EUROPE

4-2
ENGINE
ADJUSTING THE PILOT JET
The richness of the air-fuel mixture
with the throttle open 1/4 or less can
be set by adjusting the pilot jet "1".
Effects of adjusting the pilot jet
(reference)A. Idle
B. Fully open
1. #45
2. #40
3. #42
ADJUSTING THE JET NEEDLE
GROOVE POSITION
Adjusting the jet needle "1" position
affects the acceleration when the
throttle is 1/8 to 3/4 open.
1. Too rich at intermediate speeds • Rough engine operation is felt and the engine will not pick up
speed smoothly.
Step up the jet needle clip by one
groove and move down the nee-
dle to lean out the mixture.
2. Too lean at intermediate speeds • The engine breathes hard and will
not pick up speed quickly.
Step down the jet needle clip by
one groove and move up the nee-
dle to enrich the mixture. Effects of changing the jet needle
groove position (reference)
A. Idle
B. Fully open
1. No.5 groove
2. No.3 groove
3. No.4 groove
ADJUSTING THE JET NEEDLE
The jet needle is adjusted by chang-
ing it.
The jet needle setting parts, having
the same taper angle, are available in
different straight portion diameters. a. Diameter of the straight por- tion
Effects of changing the jet needle
(reference)
(Diameter of the straight portion)
Changing the diameter of the straight
portion adjusts the air-fuel mixture
when the throttle is 1/8 to 1/4 open. A. Idle
B. Fully open
RELATIONSHIP WITH THROTTLE
OPENING
The flow of the fuel through the car-
buretor main system is controlled by
the main jet and then, it is further reg-
ulated by the area between the main
nozzle and the jet needle.
The fuel flow relates to the diameter
of the straight portion of the jet needle
with the throttle 1/8 to 1/4 open and
relates to the clip position with the
throttle 1/8 to 3/4 open.
Therefore, the fuel flow is balanced at
each stage of throttle opening by the
combination of the jet needle straight
portion diameter and clip position. ADJUSTING THE LEAK JET
(ADJUSTING THE ACCELERATOR
PUMP)
The leak jet "1" is a setting part that
adjusts the flow of fuel discharged by
the accelerator pump. Since the ac-
celerator pump operates only when
throttle is open, the leak jet is used to
adjust a fuel mixture ratio for quick
throttle opening and is therefore dif-
ferent from other setting parts that ad-
just a fuel mixtur
e for each throttle
opening (each engine speed).
1. When the engine breathes hard in quick throttle opening, select a
leak jet having lower calibrating
No. than standard to enrich the
mixture. #70 →#65
2. When rough engine operation is
felt in quick throttle opening, se-
lect a leak jet having higher cali-
brating No. than standard to lean
out the mixture. #70
→#85
Standard pilot jet
#42
* #45
* For EUROPE
Standard clip posi- tion No.4
groove
Supplied jet needle GDEPR
* GDEPS
* For EUROPE
Standard leak jet #70
4

4-3
ENGINE
CARBURETOR SETTING PARTSMain jet Size Part
number
(-14943-)
Rich #182 4MX-94 #180 4MX-43
#178 4MX-93
#175 4MX-42
#172 4MX-92
(STD) #170 4MX-41 #168 4MX-91
#165 4MX-40
#162 4MX-90
* (STD) #160 4MX-39 Lean #158 4MX-89
Pilot jet Size Part
number
(-14948-)
Rich #50 4MX-07 #48 4MX-06
* (STD) #45 4MX-05 (STD) #42 4MX-04 Lean #40 4MX-03
Jet needle Size Part
number
(-14916-)
Rich GDEPN 5UM-BN GDEPP 5UM-BP
GDEPQ 5UM-B1 GDEPR 5UM-BRGDEPS 5UM-BS GDEPT 5UM-BT
GDEPU 5UM-BU
Lean GDEPV 5UM-BV
Leak jet Size Part
number
(-1494F-)
Rich #60 4JT-11
(STD) #70 4JT-15 #80 4JT-19
#90 4JT-23
#100 4JT-27
#110 4JT-29
Lean #120 4JT-31
* For EUROPE

4-4
ENGINE
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 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 diam
eter. 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

4-5
CHASSIS
CHASSIS
SELECTION OF THE SECONDARY
REDUCTION RATIO (SPROCKET)
ondary gear reduction ratio>
• It is generally said that the second-ary gear ratio should be reduced for
a longer straight portion of a speed
course and should be increased for
a course with many corners. Actual-
ly, however, as the speed depends
on the ground condition of the day
of the race, be sure to run through
the circuit to set the machine suit-
able for the entire course.
• In actuality, it is very difficult to
achieve settings suitable for the en-
tire course and some settings may
be sacrificed. Thus, the settings
should be matched to the portion of
the course that has the greatest ef-
fect on the race result. In such a
case, run through the entire course
while making notes of lap times to
find the best balance; then, deter-
mine the secondary reduction ratio.
• If a course has a long straight por-
tion where a machine can run at
maximum speed, the machine is
generally set such that it can devel-
op its maximum revolutions toward
the end of the straight line, with care
taken to avoid the engine over-rev-
ving.
Riding technique varies from rider to
rider and the performance of a ma-
chine also vary fr om machine to ma-
chine. Therefore, do not imitate other
rider's settings from the beginning but
choose your own setting according to
the level of your riding technique.
DRIVE AND REAR WHEEL
SPROCKETS SETTING PARTS
TIRE PRESSURE
Tire pressure should be adjust to suit
the road surface condition of the cir-
cuit.
• Under a rainy, muddy, sandy, or slippery condition, the tire pressure
should be lower for a larger area of
contact with the road surface. • Under a stony or hard road condi-
tion, the tire pressure should be
higher to prevent a flat tire.
FRONT FORK SETTING
The front fork setting should be made
depending on the rider's feeling of an
actual run and the circuit conditions.
The front fork setting includes the fol-
lowing three factors:
1. Setting of air spring characteris- tics
• Change the fork oil level.
2. Setting of spring preload • Change the spring.
• Install the adjustment washer.
3. Setting of damping force • Change the compression damp-
ing.
• Change the rebound damping. The spring acts on the load and
the damping force acts on the
cushion travel speed.
CHANGE IN LEVEL AND
CHARACTERISTICS OF FORK OIL
Damping characteristic near the final
stroke can be changed by changing
the fork oil amount.
Adjust the oil level in 5 mm (0.2 in)
increments or decrements. Too
low oil level caus es the front fork
to produce a noise at full rebound
or the rider to feel some pressure
on his hands or body. Alternative-
ly, too high oil level will develop
unexpectedly early oil lock with
the consequent shorter front fork
travel and deteriorated perfor-
mance and characteristics. There-
fore, adjust the front fork within
the specified range.
Secondary reduction ratio =
Number of rear wheel sprocket
teeth/Number of drive sprocket
teeth
Standard secondary
reduction ratio 50/13
(3.846)
* 47/14
(3.357)
* For EUROPE
Part
name Size Part number
Drive
sprocket
"1" (STD) 13T 9383B-13218
** (STD) * 14T 9383B-14222
Rear
wheel
sprocket
"2" ** (STD) ** 47T 1C3-25447-00 48T 5GS-25448-50
* 48T 1C3-25448-00
* 49T 1C3-25449-00
(STD) 50T 5TJ-25450-80
* (STD) * 50T 1C3-25450-00 * 51T 1C3-25451-0052T 5TJ-25452-80
* 52T 1C3-25452-00
* For AUS and NZ
** For EUROPE
Standard tire pressure:100 kPa (1.0 kgf/cm
2,
15 psi)
Extent of adjustment: 60–80 kPa (0.6–0.8 kgf/
cm
2, 9.0–12 psi)
Extent of adjustment: 100–120 kPa (1.0–1.2
kgf/cm
2, 15–18 psi)
Standard oil level: 132 mm (5.20 in)
Extent of adjustment: 95–150 mm (3.74–5.91
in)
From top of outer tube
with inner tube and
damper rod fully com-
pressed without
spring.

4-7
CHASSIS
CHOOSING SET LENGTH
1. Place a stand or block under the engine to put the rear wheel
above the floor, and measure the
length "a" between the rear wheel
axle center and the rear fender
holding bolt.
2. Remove the stand or block from the engine and with a rider astride
the seat, measure the sunken
length "b" between the rear wheel
axle center and the rear fender
holding bolt.
3. Loosen the locknut "1" and make adjustment by turning the spring
adjuster "2" to achieve the stan-
dard figure from the subtraction of
the length "b" from the length "a".
• If the machine is new and after it is broken in, the same set length of
the spring may change because of
the initial fatigue, etc. of the spring.
Therefore, be sure to make reeval-
uation.
• If the standard figure cannot be
achieved by adjusting the spring
adjuster and changing the spring
set length, replace the spring with
an optional one and make re-ad-
justment.
SETTING OF SPRING AFTER
REPLACEMENT
After replacement, be sure to adjust
the spring to the set length [sunken
length 90–100 mm (3.5–3.9 in)] and
set it.
1. Use of soft spring• Set the soft sp ring for less re-
bound damping to compensate
for its less spring load. Run with
the rebound damping adjuster
one or two clicks on the softer
side and readjust it to suit your
preference.
2. Use of stiff spring • Set the soft spri ng for more re-
bound damping to compensate
for its greater spring load. Run
with the rebound damping adjust-
er one or two clicks on the stiffer
side and readjust it to suit your
preference.
Adjusting the rebound damping will
be followed more or less by a change
in the compression damping. For cor-
rection, turn th e low compression
damping adjuster on the softer side.
When using a rear shock absorber
other than currently installed, use
the one whose overall length "a"
does not exceed the standard as it
may result in faul ty performance.
Never use one whose overall
length is greater than standard.
REAR SHOCK ABSORBER
SETTING PARTS
• Rear shock spring "1"
The I.D. color "a" is marked at the end
of the spring.
• Extent of adjustment (spring length)
Standard figure:
90–100 mm (3.5–3.9 in)
Length "a" of standard
shock:
488.5 mm (19.23 in)
TY
PE SPR
ING
RAT E SPRIN
G
PART NUM-
BER (-
22212-) I.D.
COLOR/ POINT SPRI
NG
FRE E
LEN
GTH
SO FT 4.3 5UN-00 Brown/1 260
4.5 5UN-10 Green/1 260
4.7 5UN-20 Red/1 260
4.9 5UN-30 Black/1 260
5.1 5UN-40 Blue/1 260
ST D 5.3 5UN-50 Yellow/
1 260
STI FF 5.5 5UN-60 Pink/1 260
5.7 5UN-70 White/1 260
SPRING FREE
LENGTH EXTENT OF AD-
JUSTMENT "b"
260 mm (10.24 in) 238.5–258.5
mm (9.39–10.18 in)

5-1
RADIATOR
ENGINE
RADIATOR
REMOVING THE RADIATOROrder Part name Q'ty Remarks Drain the coolant. Refer to "CHECKING THE COOLANT" sec-
tion in the CHAPTER 3.
Seat, fuel tank and left side cover Refer to "REMOVING THE SEAT, FUEL
TANK AND SIDE COVERS" section in the
CHAPTER 3.
1 Radiator guard 2
2 Radiator hose clamp 10 Only loosening.
3 Radiator hose 1 1
4 Left radiator 1
5 Radiator hose 3 1
6 Radiator pipe 2 1
7 Radiator hose 5 1
8 Catch tank hose 1
9 Right radiator 1
10 Radiator hose 2 1
11 Radiator hose 4 1
5