2002 YAMAHA YZ250F fuel

5 - 45
CHAS
11. Adjust:
Clutch lever free play
Refer to “CLUTCH ADJUSTMENT”
section in the CHAPTER 3.
Decompression lever free play
Refer to “DECOMPRESSION LEVER
ADJUSTMENT” section in the CHAP-
TER 3.
12. Clamp the clamp portion a of the num-
ber plate to the handlebar.
13. Insert the end of the fuel tank breather
hose 1 into the hole of the number plate.
HANDLEBAR

5 - 58
CHASREAR SHOCK ABSORBER
EC580000
REAR SHOCK ABSORBER
Extent of removal:1 Rear shock absorber removal2 Rear shock absorber disassembly
Extent of removal Order Part name Q’ty Remarks
Preparation for removalREAR SHOCK ABSORBER
REMOVAL
Hold the machine by placing the
suitable stand under the engine.
WARNING
Support the machine securely so there is nodanger of it falling over.
Seat and fitting band Refer to “SEAT, FUEL TANK AND SIDE
COVERS” section.
Silencer Refer to “EXHAUST PIPE AND
SILENCER” section in the CHAPTER 4.
1 Bolt (reservoir tank) 1
2 Clamp (air cleaner joint) 1 Only loosening.
3 Rear frame 1
4 Bolt (rear shock absorber-relay
arm)1 Hold the swingarm.
5 Bolt (rear shock absorber-frame) 1
6 Rear shock absorber 1
7 Locknut 1 Only loosening.
8 Adjuster 1 Only loosening.
9 Lower spring guide 1
1
2

–+ELEC

6 - 3
IGNITION SYSTEM
EC620000
IGNITION SYSTEM
INSPECTION STEPS
Use the following steps for checking the possibility of the malfunctioning engine being attributable to
ignition system failure and for checking the spark plug which will not spark.
*marked: Only when the ignition checker is used.
NOTE:


Remove the following parts before inspection.
1) Seat
2) Fuel tank

Use the following special tools in this inspection.
Dynamic spark tester:
YM-34487
Ignition checker:
90890-06754
Pocket tester:
YU-3112-C/90890-03112
Spark gap test*Clean or replace
spark plug.
Check entire ignition
system for connection.Repair or replace.
Check engine stop switch. Replace.
Check ignition coil. Primary coil Replace.
Secondary coil Replace.
Check CDI magneto. Pickup coil Replace.
Charging coil Replace.
Check neutral switch. Repair or replace.
Replace CDI unit.
No spark
OK
OK
OK
OK
OK
Spark
No good
No good
No good
No good
No good
No good
No good

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 - 3
TUN

SETTING
Effects of the setting parts on the throttle
valve opening
Å
Closed
ı
Fully open
1
Pilot screw/pilot jet
2
Throttle valve cutaway
3
Jet needle
4
Main jet
4/4
3/4
1/2
1/4
1/8
0ı
Å
1/2 3/4 1/4 1/8
1
2
3
4
Åı
Main system
The FLATCR carburetor has a primary main
jet. This type of main jet is perfect for racing
motorcycles since it supplies an even flow of
fuel, even at full load. The main air jet has
almost no effect on the air-fuel mixture. Use
the main jet and the jet needle to set the car-
buretor.
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 - 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)
Å
Idle
ı
Fully open
Standard main jet #178
1/4 1/2 3/4
+10%
#178
–10%
#188
#168
Åı

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.
*For EUROPE
NOTE:


If the engine idling speed fluctuates, turn the
pilot screw only 1/2 of a turn in either direc-
tion.

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)
Å
Idle
ı
Fully open
1
2-1/8 turns out
2
1-1/8 turns out
3
1-5/8 turns out
Standard pilot
screw position1-5/8
*1-1/2
(example)
1/4 1/2 3/4
+5%
–5%
1
23
Åı

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 #40