2000 YAMAHA YZ426F check engine

6 - 3
–+ELEC

IGNITION SYSTEM
EC620000
IGNITION SYSTEM
EC621001
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:

l
Remove the following parts before inspection.
1) Seat
2) Fuel tank
l
Use the following special tools in this inspection.
Spark gap test*Clean or replace
spark plug.
Check entire ignition
system for connection.Repair or replace.
Check “ENGINE STOP”
button.Replace.
Check ignition coil. Primary coil Replace.
Secondary coilReplace.
Check CDI magneto. Pick-up coil Replace.
Source coil Replace.
Check neutral switch. Repair or replace.
Replace CDI unit.
Dynamic spark tester:
YM-34487
Ignition checker:
90890-06754
Pocket tester:
YU-03112/90890-03112
No Spark
OK
OK
OK
OK
Spark
No good
No good
No good
No good
No good
No good
No good
OK

6 - 4
–+ELEC

IGNITION SYSTEM
EC622001
SPARK GAP TEST
1. Disconnect the spark plug cap from spark
plug.
2. Connect the dynamic spark tester
1
(ignition checker
2
) as shown.
l
Spark plug cap
3

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Spark plug
4


For USA and CDN
õ
Except for USA and CDN
3. Kick the kick starter.
4. Check the ignition spark gap.
5. Start engine, and increase spark gap until
misfire occurs. (for USA and CDN only)
Minimum spark gap:
6.0 mm (0.24 in)

õ

EC624000
COUPLERS AND LEADS CONNECTION
INSPECTION
1. Check:
l
Couplers and leads connection
Rust/dust/looseness/short-circuit
®

Repair or replace.
EC625001
“ENGINE STOP” BUTTON INSPECTION
1. Inspect:
l
“ENGINE STOP” button conduct
No continuity while being pushed
®
Replace.
Continuity 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
W

´
1
FREE

6 - 7
–+ELECTPS (THROTTLE POSITION SENSOR) SYSTEM
EC69000
TPS (THROTTLE POSITION SENSOR) SYSTEM
EC691000
INSPECTION STEPS
If the TPS will not operate, use the following inspection steps.
*marked: Refer to “IGNITION SYSTEM” section.
NOTE:
lRemove the following parts before inspection.
1) Seat
2) Fuel tank
lUse the following special tools in this inspection.
Check entire ignition
system for connection.Repair or replace.
Check TPS. TPS coil Replace.
*Check CDI magneto. Source coil Replace.
Check CDI unit.TPS input
voltageReplace.
Pocket tester:
YU-03112/90890-03112Inductive tachometer:
YU-8036-1
Engine tachometer:
90890-03113
No good
No good
No good
No good
OK
OK
OK

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.
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 com-
ponents 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.
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.
SETTING
7

7 - 9
TUN
EC71D021
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 mixtureIncrease main jet calibration no.
(Gradually)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
Accelerator pump
At full throttle
Speed pick-up stops
Slow speed pick-up
Slow response
Sooty spark plug
Rich mixtureDecrease main jet calibration no.
(Gradually)Discoloration of spark plug ®
If tan color, it is in good condition.
If cannot be corrected:
Clogged air cleaner
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.
Accelerator pump (except for rich mix-
ture symptom). Rich mixture Raise jet needle clip position.
(1 groove up)
1/4 ~ 3/4 throttle
Hard breathing
Lack of speedLower jet needle clip position.
(1 groove down)
1/4 ~ 1/2 throttle
Slow speed pick-up
Poor accelerationRaise jet needle clip position.
(1 groove up)
Closed to 1/4 throttle
Hard breathing
Speed downUse jet needle with a smaller diameter. Slow-speed-circuit passage
Clogged ® Clean.
Overflow from carburetor
Accelerator pump
Closed to 1/4 throttle
Poor accelerationUse jet needle with a larger diameter.
Raise jet needle clip position. (1 groove
up)
Poor response in the low to
intermediate speedsRaise jet needle clip position.
If this has no effect, lower the jet needle
clip position.
Poor response when throttle
is opened quicklyCheck 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 that the accelerator pump oper-
ates smoothly.Check air cleaner for fouling.
Jet needleClip
Groobe 7 Groobe 6
Groobe 5
Groobe 4
Groobe 3
Groobe 2
Groobe 1
Leaner
­
(Standard)
¯
Richer
*
SETTING