2001 YAMAHA WR 250F engine

6 - 9
–+ELECTHROTTLE POSITION SENSOR SYSTEM
THROTTLE POSITION SENSOR
REPLACEMENT AND ADJUSTMENT
1. Remove:
lThrottle position sensor coupler
lScrew (throttle position sensor) 1
lWasher 2
lThrottle position sensor 3
NOTE:
Loosen the screws (throttle position sensor)
using the T20 bit (tamper resistant fastener
type).
2. Replace:
lThrottle position sensor
3. Install:
lThrottle position sensor 1
lWasher 2
lScrew (throttle position sensor) 3
NOTE:
lAlign the slot a in the throttle position sen-
sor with the projection b on the carburetor.
lTemporarily tighten the screws (throttle posi-
tion sensor).
4. Adjust:
lEngine idling speed
Refer to “ENGINE IDLE SPEED
ADJUSTMENT” section in the CHAP-
TER 3.
5. Calculate:
lThrottle position sensor coil resistance
at idle speed

If the throttle position sensor coil resistance is
5 kW, then the throttle position sensor coil
resistance at idle speed is:
5 kW ´ (0.13 ~ 0.15) = 650 ~ 750 W
Refer to “THROTTLE POSITION SEN-
SOR COIL INSPECTION” section
about the throttle position sensor coil
resistance. Throttle position sensor coil resistance
at idle speed:
Throttle position sensor coil resis-
tance ´ (0.13 ~ 0.15)

6 - 10
–+ELECTHROTTLE POSITION SENSOR SYSTEM
6. Adjust:
lThrottle position sensor coil resistance
at idle speed
Adjustment steps:
Adjust the angle of the throttle position sen-
sor 1 to obtain the resistance calculated
(example: 650 ~ 750 W)
Tester (+) lead ® Yellow lead 2
Tester (–) lead ® Black lead 3
7. Tighten:
lScrew (throttle position sensor) 1
NOTE:
Tighten the screws (throttle position sensor)
using the T20 bit (tamper resistant fastener
type).
8. Install:
lThrottle position sensor coupler
THROTTLE POSITION SENSOR INPUT
VOLTAGE INSPECTION
1. Disconnect the throttle position sensor
coupler.
2. Start the engine.
3. Inspect:
lThrottle 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

–+ELEC
6 - 11
LIGHTING SYSTEM
LIGHTING SYSTEM
INSPECTION STEPS
Refer to the following flow chart when inspecting the ignition system for possible problems.
NOTE:
lReplace the bulb and/or bulb socket.
1) Seat
2) Fuel tank
lUse 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-08036-B
Engine tachometer:
90890-03113
OK
OK
OK
OK
No good
No good
Improperly connected
No good
No good

6 - 12
–+ELECLIGHTING SYSTEM
LIGHT SWITCH INSPECTION
1. Inspect:
lLight 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
W ´ 1
OFF
CDI MAGNETO INSPECTION
1. Inspect:
lLighting 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:
lOut-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 W at
20 ˚C (68 ˚F)W ´ 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
l
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.
l
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

l
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.
l
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

7 - 2
TUN
CAUTION:

l
The carburetor is extremely sensitive to
foreign matter (dirt, sand, water, etc.).
During installation, do not allow foreign
matter to get into the carburetor.
l
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.
l
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.
l
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:
l
Higher temperature expands the air with its
resultant reduced density.
l
Higher humidity reduces the amount of oxy-
gen in the air by so much of the water vapor
in the same air.
l
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
.
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)

Idle
õ
Fully open
Standard main jet #170
1/4 1/2 3/4
+10%
#170
–10%
#180
#160
õ

7 - 5
TUN

SETTING
Pilot screw adjustment
The richness of the air-fuel mixture with the
throttle fully closed to 1/4 open can be set by
turning the pilot screw
1
. Turning in the pilot
screw will make the mixture lean at low
speeds, and turning it out will enrich it.
NOTE:

l
If the engine idling speed fluctuates, turn the
pilot screw only 1/2 of a turn in either direc-
tion.
l
To optimize the fuel flow at a smaller throttle
opening, each machine’s pilot screw has
been individually set at the factory. Before
adjusting the pilot screw, turn it in fully and
count the number of turns. Record this num-
ber as the factory-set number of turns out.
Effects of adjusting the pilot screw (reference)

Idle
õ
Fully open
1
2-1/4 turns out
2
1-1/4 turns out
3
1-3/4 turns out
Standard pilot
screw position1-3/4
(example)
1/4 1/2 3/4
+5%
–5% AB
1
23

Pilot jet adjustment
The richness of the air-fuel mixture with the
throttle open 1/4 or less can be set by adjust-
ing the pilot jet
1
.
Standard pilot jet #42