2002 YAMAHA WR 250F engine

6 - 8
–+ELECTHROTTLE POSITION SENSOR SYSTEM
HANDLING NOTE
CAUTION:
Do not loosen the screws (throttle position
sensor) 1 except when changing the throt-
tle position sensor due to failure because it
will cause a drop in engine performance.
EC624000
COUPLERS AND LEADS CONNECTION
INSPECTION
1. Check:
Couplers and leads connection
Rust/dust/looseness/short-circuit →
Repair or replace.
THROTTLE POSITION SENSOR COIL
INSPECTION
1. Inspect:
Throttle position sensor coil resistance
Out of specification → Replace.
Tester (+) lead → Blue lead 1
Tester (–) lead → Black lead 2
Throttle posi-
tion sensor coil
resistanceTester selector
position
4 ~ 6 kΩ at
20 ˚C (68 ˚F)kΩ × 1
2. Loosen:
Throttle stop screw 1
NOTE:
Turn out the throttle stop screw until the throt-
tle shaft is in the full close position.

6 - 10
–+ELECTHROTTLE POSITION SENSOR SYSTEM
5. Adjust:
Engine idling speed
Refer to “ENGINE IDLING SPEED
ADJUSTMENT” section in the CHAP-
TER 3.
6. Insert the thin electric conductors 2
(lead) into the throttle position sensor
coupler 1, as shown, and connect the
tester to them.
CAUTION:
Do not insert the electric conductors
more than required because it may
reduce the waterproof function of the
coupler.
Make sure that a short-circuit does not
develop between the terminals because it
may cause damage to electrical compo-
nents.Tester (+) lead → Yellow lead 3
Tester (–) lead → Black lead 4
7. Start the engine.
8. Adjust:
Throttle position sensor output voltage
Adjustment steps:
Adjust the installation angle of the throttle
position sensor 1 to obtain the specified
output voltage.
NOTE:
Measure the output voltage accurately with a
digital electronic voltmeter that gives an easy
reading of a small voltage.
Throttle posi-
tion sensor
output voltageTester selector
position
0.58 ~ 0.78 V DCV

6 - 11
–+ELEC
9. Tighten:
Screw (throttle position sensor) 1
NOTE:
Tighten the screws (throttle position sensor)
using the T20 bit (tamper resistant fastener
type).
10. Stop the engine.
THROTTLE POSITION SENSOR INPUT
VOLTAGE INSPECTION
1. Disconnect the throttle position sensor
coupler.
2. Start the engine.
3. Inspect:
Throttle position sensor input voltage
Out of specification → Replace the CDI
unit.
Tester (+) lead → Blue lead 1
Tester (–) lead → Black/Blue lead 2
Throttle posi-
tion sensor
input voltageTester selector
position
4 ~ 6 V DCV-20
THROTTLE POSITION SENSOR SYSTEM

–+ELEC
6 - 12
LIGHTING SYSTEM
LIGHTING SYSTEM
INSPECTION STEPS
Refer to the following flow chart when inspecting the ignition system for possible problems.
NOTE:
Replace the bulb and/or bulb socket.
1) Seat
2) Fuel tank
Use the following special tool.
Check the bulb and bulb
socket.Replace the bulb and/
or bulb socket.
Check the light switch. Replace.
Check the CDI magneto. Lighting coil Replace.
Check the entire lighting sys-
tem proper for connections.Repair or replace.
Check the voltage regula-
tor.Out-put volt-
ageReplace.
Pocket tester:
YU-3112-C/90890-03112Inductive tachometer:
YU-8036-B
Engine tachometer:
90890-03113
OK
OK
OK
OK
No good
No good
Improperly connected
No good
No good

6 - 13
–+ELECLIGHTING SYSTEM
LIGHT SWITCH INSPECTION
1. Inspect:
Light switch continuity
No continuous while being → Replace.
Continuous while being OFF → Replace. Tester (+) lead → Yellow lead 1
Tester (–) lead → Blue lead 2
Y
1L
2Tester selec-
tor position
Ω × 1
OFF
CDI MAGNETO INSPECTION
1. Inspect:
Lighting coil resistance
Out of specification → Replace.
VOLTAGE REGULATOR INSPECTION
1. Start the engine.
2. Turn on the headlight and taillight by turn-
ing on the light switch.
3. Inspect:
Out-put voltage
Out of specification → Replace voltage
regulator. Tester (+) lead → Yellow lead 1
Tester (–) lead → Black lead 2
Lighting coil
resistanceTester selector
position
0.16 ~ 0.24 Ω at
20 ˚C (68 ˚F)Ω × 1
Tester (+) lead → Blue lead 1
Tester (–) lead → Black lead 2
Out-put voltageTester selector
position
13.3 ~ 14.3 V at
5,000 r/minACV-20
BL
1
2
1
2

7 - 1
TUN

EC700000
TUNING
EC710000
ENGINE
Carburetor setting

The air/fuel mixture will vary depending on
atmospheric conditions. Therefore, it is nec-
essary to take into consideration the air pres-
sure, ambient temperature, humidity, etc.,
when adjusting the carburetor.

Perform a test run to check for proper engine
performance (e.g., throttle response) and
spark plug(-s) discoloration or fouling. Use
these readings to determine the best possi-
ble carburetor setting.
NOTE:

It is recommended to keep a record of all car-
buretor settings and external conditions (e.g.,
atmospheric conditions, track/surface condi-
tions, lap times) to make future carburetor set-
ting easier.
WARNING


The carburetor is a part of the fuel line.
Therefore, be sure to install it in a well-
ventilated area, away from flammable
objects and any sources of fire.

Never look into the carburetor intake.
Flames may shoot out from the pipe if the
engine backfires while it is being started.
Gasoline may be discharged from the
accelerator pump nozzle when the carbu-
retor has been removed.
SETTING

7 - 2
TUN
CAUTION:


The carburetor is extremely sensitive to
foreign matter (dirt, sand, water, etc.).
During installation, do not allow foreign
matter to get into the carburetor.

Always handle the carburetor and its
components carefully. Even slight
scratches, bends or damage to carbure-
tor parts may prevent the carburetor from
functioning correctly. Carefully perform
all servicing with the appropriate tools
and without applying excessive force.

When the engine is stopped or when
riding at no load, do not open and close
the throttle unnecessarily. 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 richness or lean-
ness of the air/fuel mixture. Therefore, refer
to the above table for mixture settings.
That is:

Higher temperature expands the air with its
resultant reduced density.

Higher humidity reduces the amount of oxy-
gen in the air by so much of the water vapor
in the same air.

Lower atmospheric pressure (at a high alti-
tude) reduces the density of the air.
Air temp. HumidityAir
pressure
(altitude)Mixture Setting
High High Low (high) Richer Leaner
Low Low High (low) Leaner Richer
SETTING

7 - 4
TUN

SETTING
1
Jet needle
2
Main air jet
3
Pilot air jet
4
Needle jet
5
Main jet
6
Pilot jet
7
Pilot screw
Main jet adjustment
The richness of the air-fuel mixture at full throt-
tle can be set by changing the main jet
1
.
* For EUROPE
** For CDN, ZA, AUS and NZ
If the air-fuel mixture is too rich or too lean, the
engine power will drop, resulting in poor accel-
eration.
Effects of changing the main jet (reference)
Å
Idle
ı
Fully open
Standard main jet#175
* #172
** #170
1/4 1/2 3/4
+10%
#175
–10%
#185
#165
Åı