2005 YAMAHA YZ250F engine

6 - 4
–+ELECIGNITION SYSTEM
SPARK GAP TEST
1. Disconnect the spark plug cap from spark
plug.
2. Remove the ignition coil cap.
3. Connect the dynamic spark tester 1 (igni-
tion checker 2) as shown.
Ignition coil 3
Spark plug 4
ÈFor USA and CDN
ÉExcept for USA and CDN
4. Kick the kickstarter crank.
5. Check the ignition spark gap.
6. Start engine, and increase spark gap until
misfire occurs. (for USA and CDN only)
Minimum spark gap:
6.0 mm (0.24 in)
È
É
COUPLERS, LEADS AND IGNITION COIL
CONNECTION INSPECTION
1. Check:
Couplers and leads connection
Rust/dust/looseness/short-circuit →
Repair or replace.
Ignition coil and spark plug as they are fit-
ted
Push in the ignition coil until it closely
contacts the spark plug hole in the cylin-
der head cover.
ENGINE STOP SWITCH INSPECTION
1. Inspect:
Engine stop switch conduct
No continuous while being pushed → Replace.
Continuous while being freed → Replace. Tester (+) lead
→ Black/White lead
1
Tester (–) lead
→ Black lead
2
B/W
1 B
2 Tester selec-
tor position
PUSH IN
Ω
× 1
FREE
B/W B
11 2 2

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.
1
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
4. Install:
Throttle position sensor coupler
5. Adjust:
Engine idling speed
Refer to “ENGINE IDLING SPEED
ADJUSTMENT” section in the CHAPTER
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.
7. Start the engine.Tester (+) lead
→ Yellow lead
3
Tester (–) lead
→ Black lead
4
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 out-
put 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 T25 bit.
1
10. Stop the engine.
THROTTLE POSITION SENSOR INPUT
VOLTAGE INSPECTION
1. Disconnect the throttle position sensor cou-
pler.
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

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 carburetor
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
TUNSETTING
1Jet needle
2Pilot air jet
3Needle jet
4Main jet
5Pilot jet
6Pilot 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)
ÈIdle
ÉFully open
Standard main jet #182
1/4 1/2 3/4
+10%
#182
–10%
#185
#180
ÈÉ

7 - 5
TUNSETTING
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.
* Except for USA
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 (refer-
ence)
ÈIdle
ÉFully open
12-3/8 turns out
21-3/8 turns out
31-7/8 turns out
Standard pilot screw
position1-7/8
(example)
* 2-1/8
1
1/4 1/2 3/4
+5%
–5%
1
23
ÈÉ
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