engine YAMAHA YZ125LC 2003 Manual Online

6-2
ELECZÜNDANLAGE
*Zündkerze reinigen oder
erneuern.
Primärspule
Sekundärspule
Erneuern oder reparieren.
Zündfunke
Die Anschlüsse der gesamten
Zündanlage kontrollieren.
Test der Zündfunkenstrecke
Nicht in Ordnung
Nicht in Ordnung
Nicht in Ordnung
Nicht in Ordnung
Nicht in Ordnung
Nicht in Ordnung Kein Zündfunke
Den Motorstoppknopf „ENGINE
STOP“ kontrollieren.
Aufnahmespule
Stromspule
Zündspule kontrollieren.
CDI-Magnetzünder kontrollieren.
CDI-Einheit erneuern.
Erneuern.
Erneuern.
Erneuern.
Erneuern.
Erneuern.
In Ordnung
In Ordnung
In Ordnung
In Ordnung
ZÜNDANLAGE
PRÜFVORGÄNGE
Durch folgende Schritte auf eine mögliche Störung des Motors durch einen Defekt im Zündsystem prü-
fen und Ausfindigmachen einer aussetzenden Zündkerze.
Markierung * : Nur wenn das Zündungsprüfgerät verwendet wird.
HINWEIS:
9Die folgenden Teile vor Beginn der Störungsbeseitigung ausbauen.
1) Sitz
2) Kraftstofftank
9Die folgenden Spezialwerkzeuge für die Störungsbeseitigung verwenden.
Dynamischer Zündfunkenprüfer:
YM-34487
Zündungsprüfgerät:
90890-06754Taschenprüfgerät:
YU-3112-C/90890-03112
Erneuern.Den Zündkerzenstecker
kontrollren.Nicht in Ordnung
In Ordnung

6-3
ELECIGNITION SYSTEM
EC622001
SPARK GAP TEST
1. Disconnect the spark plug cap from spark
plug.
2. Connect the dynamic spark tester 1(igni-
tion checker 2) as shown.
9Spark plug cap 3
9Spark plug 4
AFor USA and CDN
BExcept 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)
EC624000
COUPLERS AND LEADS CONNECTION
INSPECTION
1. Check:
9Couplers and leads connection
Rust/ Dust/ Looseness/Short-circuit 
Repair or replace.
EC625002
“ENGINE STOP” BUTTON INSPECTION
1. Inspect:
9“ENGINE STOP” button conduct
Not continuous while being pushed Replace.
Continuous while being freed Replace.
Minimum spark gap:
6.0 mm (0.24 in)
Tester (+) leadBlack/White lead 1
Tester (–) lead
Black lead 2
B/W B
12
PUSH IN
FREE
Tester selector
position
Ω× 1
å
∫

6-3
ELEC
SYSTEME D’ALLUMAGE
ZÜNDANLAGE
SISTEMA DI ACCENSIONE
TEST DER ZÜNDFUNKENSTRECKE
1. Den Zündkerzenstecker von der
Zündkerze abtrennen.
2. Den dynamischen Zündfunken-
prüfer 1(Zündungs-Prüfgerät
2) gemäß Abbildung ansch-
ließen.
9Zündkerzenstecker 3
9Zündkerze 4
AFür USA und CDN
BAußer USA und CDN
3. Den Motor mit Hilfe des Kickstar-
ters durchdrehen.
4. Die Zündfunkenstrecke kontrollie-
ren.
5. Den Motor anlassen und die
Zündfunkenstrecke vergrößern,
bis es zu Fehlzündungen kommt.
(nur für USA und CDN)
INSPEKTION DES ANSCHLUSSES
DER KUPPLUNGEN UND KABEL
1. Kontrollieren:
9Anschluß deren Kupplungen
und Kabel
Rost/Staub/Lockerheit/Kurz-
schlußReparieren oder
erneuern.
INSPEKTION DES MOTORSTOPP-
KNOPFES „ENGINE STOP“
1. Prüfen:
9Kontakt des Motorstoppknop-
fes „ENGINE STOP“
Kein Stromdurchgang, wenn gedrückt
Erneuern.
Stromdurchgang, wenn freigelassen
Erneuern.ESSAI DE L’INTERVALLE
D’ETINCELLEMENT
D’ ALLUMAGE
1. Déconnecter le capuchon de bou-
gie de la bougie.
2. Connecter le testeur dynamique
d’étincelle 1(testeur d’allumage
2) comme indiqué.
9Capuchon de bougie 3
9Bougie 4
A Pour USA et CDN
BExcepté pour USA et CDN
3. Actionner la pédale de kick.
4. Contrôler l’écartement d’étincelle
d’allumage.
5. Démarrer le moteur et augmenter
l’intervalle d’étincellement jusqu’à
ce qu’un raté se produise. (unique-
ment USA et CDN)
VERIFICATION DE CONNEXION
DES COUPLEURS ET FILS
1. Contrôler:
9Connexion des coupleurs et fils
Rouille/poussière/desserré/court
-circuit→
Réparer ou changer.
VERIFICATION DU BOUTON
D’ARRET DU MOTEUR “ENGINE
STOP”.
1. Examiner:
9Conductibilité du bouton d’arrêt
du moteur “ENGINE STOP”
Pas de continuité lorsqu’enfoncé 
Changer.
Continuité lorsque relâché  Changer.
IC622001
TEST DELLA DISTANZA TRA
GLI ELETTRODI
1. Scollegare la calotta della candela
dalla candela.
2. Collegare il tester dinamico delle
scintille 1(apparecchio per il
controllo dell’accensione 2) come
illustrato.
9Calotta della candela 3
9Candela 4
APer USA e CDN
BTranne che per USA e CDN
3. Azionare la pedivella della messa
in moto.
4. Controllare la distanza tra gli elet-
trodi dell’accensione.
5. Avviare il motore e aumentare la
distanza tra gli elettrodi finché non
si verifica uno scatto a vuoto. (sol-
tanto per USA e CDN)
IC624000
ISPEZIONE DEL
COLLEGAMENTO DI
ACCOPPIATORI E CONDUTTORI
1. Controllare:
9Collegamento di accoppiatori e
conduttori
Ruggine/Polvere/Allentamen-
to/Cortocircuito Ripararli o
sostituirli.
IC625002
ISPEZIONE DEL PULSANTE
“ARRESTO MOTORE”
1. Ispezionare:
9Comportamento del pulsante
“ARRESTO MOTORE”
Assenza di continuità mentre è premuto
Sostituirlo.
Continuità mentre viene rilasciato 
Sostituirlo.B/W B
Posizione12del seletto
re del tester
PREM-
11UTO
RILAS-Ω× ×

1
CIATO
Conduttore (+) del tester 
Conduttore Nero/Bianco 1
Conduttore (–) del tester 
Conduttore Nero 2
Distanza minima tra gli
elettrodi:
6,0 mm (0,24 in)
Ecartement d’étincelle
minimum:
6,0 mm (0,24 in)
Fil (+) de testeur 
Fil noir/blanc 1
Fil (–) de testeur 
Fil noir 2
B/W BPosition de12sélecteur du
testeur
ENFO-
11NCE
RELA-Ω× ×

1
CHE
Min. Zündfunkenstrecke:
6,0 mm (0,24 in)
Positives (+) Kabel des Prüfgerätes
Schwarz/Weiß Kabel 1
Negatives (–) Kabel des Prüfgerätes
Schwarz Kabel 2
B/W BPosition des12Wahlschaltersam Prüfgerät
GEDR-11ÜCKT
FREIGE-Ω×
×

1
LASSEN

7-1
TUNSETTING
EC700000
TUNINGEC710000
ENGINEEC711001
Carburetor setting
9The role of fuel is to cool the engine, and in the
case of a 2-stroke engine, to lubricate the
engine in addition to power generation.
Accordingly, if a mixture of air and fuel is too
lean, abnormal combustion will occur, and
engine seizure may result. If the mixture is too
rich, spark plugs will get wet with oil, thus mak-
ing it impossible to bring the engine into full play
or if the worst comes to the worst, the engine
may stall.
9The richness of the air-fuel mixture required for
the engine will vary with atmospheric conditions
of the day and therefore, the settings of the car-
buretor must be properly suited to the atmos-
pheric conditions (air pressure, humidity and
temperature).
9Finally, the rider himself must make a test-run
and check his machine for conditions (pick-up
of engine speed, road surface conditions) and
for the discoloration of the spark plug(s). After
taking these into consideration, he must select
the best possible carburetor settings.
It is advisable to make a note of settings,
atmospheric conditions, road surface condition,
lap-time, etc. so that the memorandum can be
used as a reference useful for future.
EC712000
Atmospheric conditions and carburetor
setting
The reason for the above tendency is that the
richness or leanness of a fuel mixture depends
on the density of the air (i.e. the concentration of
oxygen in it).
Air
Air temp. Humidity pressure Mixture Setting
(altitude)
High HighLow
Richer Leaner
(high)
Low LowHigh
Leaner Richer
(low)

7-2
TUNSETTING
That is:
9Higher temperature expands the air with its
resultant reduced density.
9Higher humidity reduces the amount of oxygen
in the air by so much of the water vapor in the
same air.
9Lower atmospheric pressure (at a high altitude)
reduces the density of the air.
EC713001
Test run
After warming up the engine equipped with the
standard type carburetor(s) and spark plug(s),
run two or three laps of the circuit and check the
smooth operation of the engine and discoloration
of spark plug(s).
åNormal
∫Over burned (too lean)
çOil fouled (too rich)
Discoloration Condition of spark plug
NormalInsulator is dry and
burnt brown.
Over burned
Insulator is whitish.
(too lean)
Oil fouled Insulator is sooty and
(too rich) wet.
A
B
C

7-5
TUNSETTING
EC718010
Jet needle groove position adjustment
Should the engine be hard to run smoothly at
intermediate speeds, the jet needle 1must be
adjusted. If the mixture is too rich or too lean at
intermediate speed operation, irregular engine
operation and poor acceleration will result.
Whether or not the richness of the mixture is
proper is hard to be determined by means of the
spark plug and therefore, it should be judged
from your feeling of actual engine operation.
*For EUROPE
1. Too rich at intermediate speeds
Rough engine operation is felt and the
engine will not pick up speed smoothly. In
this case, step up the jet needle clip by one
groove or 0.5 groove and move down the
needle to lean out the mixture.
2. Too lean at intermediate speeds
The engine breathes hard and will not pick
up speed quickly. In this case, step down the
jet needle clip by one groove or 0.5 groove
and move up the needle to enrich the mix-
ture.
Standard clip positionNo.3 groove
*No.4 groove

7-10
TUNSETTING
EC71C020
Road condition and examples of carburetor setting
EC71D040
Examples of carburetor setting depending on symptom
Symptom Setting Checking
At full throttle Increase main jet calibration No. Discoloration of spark plug

Stall at high speeds (Gradually) If tan color, it is in good condition.
* Hard breathing If can not be normalized:
Shearing noise Clogged float valve seat
Whitish spark plug Clogged fuel hose
Clogged fuel cock
Lean mixture
At full-throttle Decrease main jet calibration No. Discoloration of spark plug 
Stop of speed pick-up (Gradually) If tan color, it is in good condition.
Slow speed pick-up *In case of racing slight enrichment If not effect:
of mixture reduces engine trouble. Clogged air cleaner
Slow response Fuel overflow from carburetor
Sooty spark plug Clogged main air passage or clogged
filter
Rich mixture
Lean mixture Lower jet needle clip position.
(1 groove down)
Rich mixture Raise jet needle clip position.
(1 groove up)
1/4~3/4 throttle Lower jet needle clip position.
* Hard breathing (1 groove down)
Lack of speed
1/4~1/2 throttle Raise jet needle clip position.
Slow speed pick-up (1 groove up)
White smoke
Poor acceleration
Conditions General condition Sandy condition
Under 10°C 15~25°C Over 30°C Under 10°C 15~25°C Over 30°C
(50°F) (59~77°F) (86°F) (50°F) (59~77°F) (86°F)
Parts (Winter)(Spring, Autumn)(Summer) (Winter)(Spring, Autumn)(Summer)
Main jet #420 #410 #400 #440 #430 #420
Jet needle6CHY6-81-3 6CHY6-82-3 6CHY5-82-3 6CHY5-81-4 6CHY5-82-4 6CHY6-82-3*6CHY6-81-4 *6CHY6-82-4 *6CHY5-82-4 *6CHY5-81-5 *6CHY5-82-5 *6CHY6-82-4
Pilot jet #40 #40 #40 #40 #40 #40
Pilot air screw1-1/222
1-1/222
*1-1/2 *1-1/2 *1-1/2 *1-1/2
Leaner
(Standard)
Richer


Clip position indicates the position of jet
needle groove, to which the clip is fitted.
The position is numbered from the top.
If a change in the clip position (1
groove) is effective, try another jet nee-
dle that provides a difference of 0.5 in
the clip position.
*For EUROPE

7-11
TUNSETTING
Symptom Setting Checking
0~1/4 throttle Use jet needle having a smaller Number of turns-back 
* Hard breathing diameter. Correct properly
Speed down Overflow from carburetor
0~1/4 throttle Use jet needle having a larger
Poor acceleration diameter.
White smoke
Unstable at low speeds Lower jet needle clip position.
Pinking noise (1 groove down)
Turn in pilot air screw.
Poor response at extremely Reduce pilot jet calibration No. Dragging brake
low speed Turn out pilot air screw. Overflow from carburetor
If not effect, reverse the
above procedures.
Poor response in Raise jet needle clip position.
the range of low to If no effect, reverse the above
intermediate speeds procedures.
Poor response when Check overall settings. Check air cleaner for fouling.
throttle is opened quickly Use main jet having lower
calibration No.
Raise jet needle clip position.
(1 groove up)
If no effect, reverse the above
procedures.
Poor engine operation Turn in pilot air screw. Check throttle valve operation.
This should be taken simply for an example.
It is necessary to set the carburetor while
checking the operating conditions of the
engine and discoloration of spark plugs.
Normally, carburetor setting is made by
means of the main jet, jet needle clip position
(including one with 0.5 difference), pilot jet
and pilot air screw. If the result of setting is
still unsatisfactory, it is advisable to change
the diameter of the straight portion the jet
needle.
*In case of hard breathing, check the air vent hose for clogging.

7-12
TUNSETTING
EC71M010
Change of the heat range of spark plugs
Judging from the discoloration of spark plugs, if
they are found improper, it can be corrected by
the following two methods; changing carburetor
settings and changing the heat range of spark
plug.
9In principle, it is advisable to first use spark
plugs of standard heat range, and judging from
the discoloration of spark plugs, adjust carbure-
tor settings.
9If the calibration No. of the main jet must be
changed by ±30, it is advisable to change the
heat range of spark plugs and newly select the
proper main jet.
NOTE:
9When checking the discoloration of spark
plugs, be sure to stop the engine immediately
after a run and check.
9Avoid racing.
9When changing the heat range of spark plugs,
never attempt to change it more than ±1 rank.
9When using a spark plug other than standard,
check its heat range against the standard and
check that it is a resistance type.
9Note that even if the discoloration seems prop-
er, it may slightly vary with the spark plug
maker and oil in use.
Standard spark plugBR9EG/NGK(resistance type)

7-13
TUNSETTING
EC71X000
Selecting the rotor
If you feel the engine pick up faster depending
on the course condition, you can control it by
changing the rotor to the one with a greater iner-
tial mass.
* Inertial mass: The inertial mass of the body is
proportional to its weight and increases in pro-
portion to the square of the distance between its
rotation center and center of gravity. For the
same weight, a greater distance from its rota-
tion center produces greater inertial mass.
Greater initial mass gives a feeling of greater
torque in response to engine revolutions but is
accompanied by greater pick up resistance.
Engine characteristics are shown below in rela-
tion to each rotor:
9Standard rotor
Engine pickup is faster in the entire engine
speed range.
Pickup is fast at an extremely low engine
speed.
9Optional rotor
A feeling of torque is obtained at high engine
speed.
A broader power band is felt from mid speed to
over revs.
NOTE:
9Engine pick up differs according to the course
condition, rider, the settings, etc. Select a suit-
able rotor by comparing it with the standard
one in operation.
9It is difficult to be advantageous under all
course conditions with a change in the rotor.
Measure the lap times in accordance with race
points and select a rotor to produce a good
entire balance.
Part name Part number*Inertial mass
1 1
Standard rotor5CU-85550-11 0.0004kg•m2
2 2
Optional rotor5UN-85550-000.00045kg•m2