2011 YAMAHA YZ125LC engine

6-2
ELECTRICAL COMPONENTS AND WIRING DIAGRAM
ELECTRICAL
COMPONENTS AND
WIRING DIAGRAM
ELECTRICAL COMPONENTS
1. Engine stop switch
2. CDI unit3. Ignition coil
4. CDI magneto5. Spark plug
WIRING DIAGRAM
1. Engine stop switch
2. CDI unit
3. Ignition coil
4. CDI magneto
5. Spark plugCOLOR CODE
BBlack
O Orange
YYellow
B/R Black/Red
B/W Black/White
G/L Green/Blue
G/W Green/White
W/L White/Blue
W/R White/Red

6-3
IGNITION SYSTEM
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.
• Remove the following parts before inspection.
1. Seat
2. Fuel tank
• Use the following special tools in this inspection.
Spark gap test Spark→*Clean or replace spark plug.
No spark↓
Check entire ignition system for connection. No good→Repair or replace.
OK↓
Check engine stop switch. No good→Replace.
OK↓
Check ignition coil. (primary coil and secondary
coil)No good→
Replace.
OK↓
Check spark plug cap. No good→Replace.
OK↓
Check CDI magneto. (pickup coil and charging
coil)No good→
Replace.
OK↓
Replace CDI unit.
Dynamic spark tester:
YM-34487
Ignition checker:
90890-06754
Pocket tester:
YU-3112-C/90890-03112

6-4
IGNITION SYSTEM
SPARK GAP TEST
1. Disconnect the spark plug cap
from spark plug.
2. Connect the dynamic spark tester
"1" (ignition checker "2") as
shown.
• Ignition coil "3"
• Spark plug "4"
A. For USA and CDN
B. Except for USA and CDN
3. Kick the kickstarter crank.
4. Check the ignition spark gap.
5. Start engine, and increase spark
gap until misfire occurs. (for USA
and CDN only)
CHECKING THE COUPLERS,
LEADS AND IGNITION COIL
CONNECTION
1. Check:
• Couplers and leads connection
Rust/dust/looseness/short-circuit
→Repair or replace.CHECKING THE ENGINE STOP
SWITCH
1. Inspect:
• Engine stop switch conduction
Not conductive while it is pushed→
Replace.
Conductive while it is freed→Re-
place.
Set the tester selection position to "Ω
× 1".
CHECKING THE IGNITION COIL
1. Inspect:
• Primary coil resistance
Out of specification→Replace.
2. Inspect:
• Secondary coil resistance
Out of specification→Replace.
• Remove the spark plug cap by turn-
ing it counterclockwise and inspect.
• Install the spark plug cap by turning
it clockwise until it is tight.
CHECKING THE SPARK PLUG
CAP
1. Inspect:
• Spark plug cap
Loose connection→Tighten.
Deteriorated/damaged→Re-
place.
• Spark plug cap resistance
Out of specification→Replace.
Minimum spark gap:
6.0 mm (0.24 in)
Tester (+) lead→Black/White lead
"1"
Tester (-) lead→Black lead "2"
Result
Conductive (while the
engine stop switch is
pushed)
Tester (+) lead→Yellow lead "1"
Tester (-) lead→Black lead "2"
Primary
coil resis-
tanceTester se-
lector posi-
tion
0.24–0.36 Ω at 20 °C
(68 °F) Ω × 1
Tester (+) lead→Spark plug lead
"1"
Tester (-) lead→Yellow lead "2"
Secondary
coil resis-
tanceTester se-
lector posi-
tion
5.7–8.5 kΩat 20 °C
(68 °F) kΩ × 1
Tester (+) lead→Spark plug lead
terminal "1"
Tester (-) lead→Spark plug termi-
nal "2"
Spark plug
cap resis-
tanceTester se-
lector posi-
tion
4–6 kΩ at
20 °C (68
°F) kΩ × 1

7-1
ENGINE
TUNING
ENGINE
CARBURETOR SETTING
• The role of fuel is to cool the engine, and in the case of a 2-stroke en-
gine, to lubricate the engine in addi-
tion to power generation.
Accordingly, if a mixture of air and
fuel is too lean, abnormal combus-
tion will occur, and engine seizure
may result. If the mixture is too rich,
spark plugs will get wet with oil, thus
making it impossible to bring the en-
gine into full play or if the worst
comes to the worst, the engine may
stall.
• The richness of the air-fuel mixture
required for the engine will vary with
atmospheric conditions of the day
and therefore, the settings of the
carburetor must be properly suited
to the atmospheric conditions (air
pressure, humidity and tempera-
ture).
• Finally, 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 set-
tings.
It is advisable to make a note of set-
tings, atmospheric conditions, road
surface condition, lap-time, etc. so
that the memorandum can be used
as a reference useful for future.
ATMOSPHERIC CONDITIONS AND
CARBURETOR SETTINGS
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).
• Higher temperature expands the air with its resultant reduced density. • Higher humidity reduces the
amount of oxygen in the air by so
much of the water vapor in the
same air.
• Lower atmospheric pressure (at a high altitude) reduces the density of
the air.
TEST RUN
After warming up the engine
equipped with the standard type car-
buretor(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).
A. Normal
B. Over burned (too lean)
C. Oil fouled (too rich)
EFFECT OF SETTING PARTS IN
RELATION TO THROTTLE VALVE
OPENING A. Closed
B. Full-open
1. Pilot air screw
2. Pilot jet
3. Jet needle
4. Diameter of straight portion
5. Clip position
6. Throttle valve
7. Main jet
ADJUSTING THE MAIN JET
The richness of air-fuel mixture with
1/2–4/4 throttle can be set by chang-
ing the main jet "1".
1. Spark plug is too hot. • Select a main jet having higher calibrating No. than standard. (To
be enriched)
2. Spark plug is wet. • Select a main jet having lower cal-
ibrating No. than standard. (To be
leaned out)
ADJUSTING THE PILOT AIR
SCREW
The richness of the air-fuel mixture
with full closed to 1/4 throttle can be
set by turning the pilot air screw "1".
Turning in the pilot air screw will en-
rich the mixture at low speeds, and
turning out it will lean out the mixture.
Air
temp . Hu-
midi- ty Air
pres- sure
(alti-
tude) Mix-
ture Set-
ting
High High Low
(high) Rich-
er Lean
er
Low Low High
(low) Lean
er Rich-
er
Discoloration Condition of spark
plug
Normal Insulator is dry
and burnt brown.
Over burned (too lean) Insulator is whit-
ish.
Oil fouled (too rich) Insulator is sooty
and wet.
Standard main jet #430
Standard pilot
air screw posi- tion 2-1/4 turns out

7-2
ENGINE
ADJUSTING THE PILOT JET
The richness of air-fuel mixture with
the throttle fully closed to 1/2 open
can be set by changing the pilot jet
"1". It is changed when adjustment
cannot be made by the pilot air screw
alone.
ADJUSTING THE JET NEEDLE
GROOVE POSITION
Should the engine be hard to run
smoothly at intermediate speeds, the
jet needle "1" must be adjusted. If the
mixture is too rich or too lean at inter-
mediate speed operation, irregular
engine operation and poor accelera-
tion 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.
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 mixture. ADJUSTING THE JET NEEDLE
On the carburetors used in the
YZ125, the main nozzle is a non dis-
assembly type, so it can not be re-
placed. Therefore,
carburetor setting
requires the change of the jet needle.
1. The jet needle setting parts, hav-
ing the same taper angle, are
available in different straight por-
tion diameters and in different ta-
per starting positions.
In the case of the same number of clip
position, changing from 6BFY43-74
to 6BFY42-74 has the same effect as
a lowering of 0.5-clip position. And in
the case of the same number of clip
position, changing from 6BFY43-74
to 6BFY44-74 has the same effect as
a rising of 0.5-clip position.
A. Difference in straight portion dia.
B. Difference in clip position
a. Reference needle
b. 0.5 richer
c. 0.5 leaner
RELATIONSHIP WITH THROTTLE
OPENING
The flow of the fuel through the car-
buretor main system is controlled by
the main jet and then, it is further reg-
ulated by the area between the main
nozzle and the jet needle. On the re-
lationship between the fuel flow and
the throttle opening, the fuel flow re-
lates to the straight portion of the jet
needle at full closed–1/8 throttle, to
the 1st tapered porti on at 1/4 throttle,
to the second tapered portion at 1/2
throttle, to the third tapered portion at
3/4 throttle and to the fourth tapered
portion at full open. Therefore, the fuel fl
ow is balanced at
each stage of throttle opening by a
combination of the jet needle diame-
ter and clip position.

A. Lean (larger diameter)
B. Rich (smaller diameter)
C. 1st taper
D. 2nd taper
E. 3rd taper
F. 4th taper
1. Full closed
2. 1/4 throttle
3. 1/2 throttle
4. 3/4 throttle
5. Full open
a. Main nozzle
Standard pilot jet
#40
*#45
*Except for USA and CDN
Standard clip position No.3 groove
Standard jet
needle 6BFY43-74
6BFY43-73
6BFY43-74
6BFY43-75
6BFY44-74
6BFY43-74
6BFY42-74
6BFY43-74-3
6BFY43-74-2
6BFY43-75-3
7

7-3
ENGINE
CARBURETOR SETTING PARTSMain jet "1" SizePart number
(-14143-)
Rich #470 137-94 #460 137-92
#450 137-90
#440 137-88
(STD) #430 137-86 #420 137-84
#410 137-82
Lean #400 137-80
Pilot jet "2" Size
Part number
(-14142-)
Rich #50 4KM-50 #47.5 4KM-47
** (STD) #45 4KM-45 #42.5 4KM-42
* (STD) #40 4KM-40 #37.5 4KM-37#35 4KM-35
#32.5 4KM-32
Lean #30 4KM-30
Throttle
valve "3" SizePart number
(-14112-)
Rich
(STD) 4.0 1C3-40 Lean 4.25 1C3-42
* For USA and CDN
** Except for USA and CDNJet nee- dle "4" SizePart
number
(-14116-)
Rich 6BFY44-72 284-K2 6BFY44-73 284-K3
6BFY44-74 284-K4
6BFY44-75 284-K5
Lean 6BFY44-76 284-K6 Rich 6BFY43-72 284-J2 6BFY43-73 284-J3
(STD) 6BFY43-74 284-J4 6BFY43-75 284-J5
Lean 6BFY43-76 284-J6 Rich 6BFY42-72 284-H2 6BFY42-73 284-H3
6BFY42-74 284-H4
6BFY42-75 284-H5
Lean 6BFY42-76 284-H6
* For USA and CDN
** Except for USA and CDN

7-4
ENGINE
ROAD CONDITION AND EXAMPLES OF CARBURETOR SETTING
A. For USA and CDN
B. Except for USA and CDN
SPECIFICATIONS OF JET NEEDLE
EXAMPLES OF CARBURETOR SETTING DEPENDING ON SYMPTOM General condition Sandy condition
Under 10°C (50°F) 15–25°C (59–
77°F) Over 30°C
(86°F) Under 10°C
(50°F) 15–25°C (59–
77°F) Over 30°C
(86°F
(Winter) (Spring, Au- tumn)(Summer) (Winter) (Spring, Au-
tumn)(Summer)
Main jet #440 #430 #420 #460 #450 #440
Jet needle 6BFY44-74-3 6BFY43-74-3 6BFY44- 74-2 6BFY43-74-4 6BFY44-74-3 6BFY43-74-3
Pilot jet A #42.5 #40 #40 #42.5 #40 #40 B #47.5 #45 #42.5 #47.5 #45 #42.5
Pilot air screw 2-1/4 2-1/4 2-1/4 2-1/4 2-1/4 2-1/4
Diameter of straight portion
ø2.72 mm
(0.1071 in) ø2.73 mm
(0.1075 in) ø2.74 mm
(0.1079 in) ø2.75 mm
(0.1083 in) ø2.76 mm
(0.1087 in)
Rich 1 richer 6BFY43-72-4 6BFY43-73-4 6BFY43-74-4 6BFY43-75-4 6BFY43-76-4
0.5 richer 6BFY44-72-3 6BFY44-73-3 6BFY44-74-3 6BFY44-75-3 6BFY44-76-3
6BFY42-72-4 6BFY42-73-4 6BFY42-74-4 6BFY42-75-4 6BFY42-76-4
STD 6BFY43-72-3 6BFY43-73-3 6BFY43-74-3 6BFY43-75-3 6BFY43-76-3
0.5 leaner 6BFY44-72-2 6BFY44-73-2 6BFY44-74-2 6BFY44-75-2 6BFY44-76-2
6BFY42-72-3 6BFY42-73-3 6BFY42-74-3 6BFY42-75-3 6BFY42-76-3
Lean 1 leaner 6BFY43-72-2 6BFY43-73-2 6BFY43-74-2 6BFY43-75-2 6BFY43-76-2
Symptom Setting Checking
At full throttle Stall at high speeds
*Hard breathing
Shearing noise
Whitish spark plug ↓
Lean mixture Increase main jet calibration no. (Gradual-
ly)
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
At full throttle Stop of speed pick-up
Slow speed pick-up
Slow response
Sooty spark plug ↓
Rich mixture Decrease main jet calibration no. (Gradual-
ly)
*In case of racing slig
ht enrichment of mix-
ture reduces engine trouble. Discoloration of spark plug
→If tan color, it
is in good condition.
If not effect: Clogged air filter
Fuel overflow from carburetor
Clogged main air passage or clogged fil-
ter

7-5
ENGINE
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, carb uretor 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.
Lean mixture Lower jet needle clip position. (1 groove
down)
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 needle that pro-
vides a difference of 0. 5 in the clip position.
Rich mixture Raise jet needle clip position. (1 groove up)
1/4–3/4 throttle
*Hard breathing
Lack of speed Lower jet needle clip position. (1 groove
down)
1/4–1/2 throttle Slow speed pick-up
White smoke
Poor acceleration Raise jet needle clip position. (1 groove up)
0–1/4 throttle *Hard breathing
Speed down Use jet needle having a smaller diameter. Number of turns-back
→Correct properly
Overflow from carburetor
0–1/4 throttle Poor acceleration
White smoke Use jet needle with a larger diameter.
Unstable at low speeds Pinking noise Lower jet needle clip position.
(1 groove down)
Turn in pilot air screw.
Poor response at extremely low
speed Reduce pilot jet calibration No.
Turn out pilot air screw.
If not effect, reverse the above procedures. Dragging brake
Overflow from carburetor
Poor response in the range of
low to intermediate speeds Raise jet needle clip position.
If no effect, reverse the above procedures.
Poor response when throttle is
opened quickly Check overall settings.
Use main jet having lower calibration no.
Raise jet needle clip position. (1 groove up)
If no effect, reverse the above procedures. Check air filter for fouling.
Poor engine operation Turn in pilot ai r screw. Check throttle valve operation.
* In case of hard breathing, check the air vent hose for clogging. Symptom Setting CheckingGroove 1
Groove 2
Groove 3
Groove 4
Groove 5
Clip
Jet needle
Leaner
(Standard)
Richer