2012 YAMAHA YZ125LC spark plugs

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
OPENINGA. 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-
tyAir
pres-
sure
(alti-
tude)Mix-
tureSet-
ting
High HighLow
(high)Rich-
erLean
er
Low LowHigh
(low)Lean
erRich-
er
DiscolorationCondition of spark
plug
NormalInsulator 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-
tion2-1/4 turns out

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, 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.
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 speedLower jet needle clip position. (1 groove
down)
1/4–1/2 throttle
Slow speed pick-up
White smoke
Poor accelerationRaise jet needle clip position. (1 groove up)
0–1/4 throttle
*Hard breathing
Speed downUse jet needle having a smaller diameter. Number of turns-back→Correct properly
Overflow from carburetor
0–1/4 throttle
Poor acceleration
White smokeUse jet needle with a larger diameter.
Unstable at low speeds
Pinking noiseLower jet needle clip position.
(1 groove down)
Turn in pilot air screw.
Poor response at extremely low
speedReduce 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 speedsRaise jet needle clip position.
If no effect, reverse the above procedures.
Poor response when throttle is
opened quicklyCheck 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 air 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 5Clip
Jet needle
Leaner
(Standard)
Richer

7-6
CHASSIS
CHANGE OF THE HEAT RANGE
OF SPARK PLUGS
Judging from the discoloration of
spark plugs, if they are found improp-
er, it can be corrected by the following
two methods; changing carburetor
settings and changing the heat range
of spark plug.
• In principle, it is advisable to first
use spark plugs of standard heat
range, and judging from the discol-
oration of spark plugs, adjust carbu-
retor settings.
• If the calibration No. of the main jet
must be changed by ±30, it is advis-
able to change the heat range of
spark plugs and newly select the
proper main jet.
• When checking the discoloration of
spark plugs, be sure to stop the en-
gine immediately after a run and
check.
• Avoid racing.
• When changing the heat range of
spark plugs, never attempt to
change it more than ±1 rank.
• When using a spark plug other than
standard, check its heat range
against the standard and check that
it is a resistance type.
• Note that even if the discoloration
seems proper, it may slightly vary
with the spark plug maker and oil in
use.
CHASSIS
SELECTION OF THE SECONDARY
REDUCTION RATIO (SPROCKET) ondary gear reduction ratio>
• It is generally said that the second-
ary gear ratio should be reduced for
a longer straight portion of a speed
course and should be increased for
a course with many corners. Actual-
ly, however, as the speed depends
on the ground condition of the day
of the race, be sure to run through
the circuit to set the machine suit-
able for the entire course.
• In actuality, it is very difficult to
achieve settings suitable for the en-
tire course and some settings may
be sacrificed. Thus, the settings
should be matched to the portion of
the course that has the greatest ef-
fect on the race result. In such a
case, run through the entire course
while making notes of lap times to
find the best balance; then, deter-
mine the secondary reduction ratio.
• If a course has a long straight por-
tion where a machine can run at
maximum speed, the machine is
generally set such that it can devel-
op its maximum revolutions toward
the end of the straight line, with care
taken to avoid the engine over-rev-
ving.
Riding technique varies from rider to
rider and the performance of a ma-
chine also vary from machine to ma-
chine. Therefore, do not imitate other
rider's settings from the beginning but
choose your own setting according to
the level of your riding technique.
DRIVE AND REAR WHEEL
SPROCKETS SETTING PARTSTIRE PRESSURE
Tire pressure should be adjust to suit
the road surface condition of the cir-
cuit.
• Under a rainy, muddy, sandy, or
slippery condition, the tire pressure
should be lower for a larger area of
contact with the road surface.
• Under a stony or hard road condi-
tion, the tire pressure should be
higher to prevent a flat tire.
FRONT FORK SETTING
The front fork setting should be made
depending on the rider's feeling of an
actual run and the circuit conditions.
The front fork setting includes the fol-
lowing three factors:
1. Setting of air spring characteris-
tics
• Change the fork oil amount.
2. Setting of spring preload
• Change the spring.
3. Setting of damping force
• Change the compression damp-
ing.
• Change the rebound damping.
The spring acts on the load and
the damping force acts on the
cushion travel speed.
CHANGE IN AMOUNT AND
CHARACTERISTICS OF FORK OIL
Damping characteristic near the final
stroke can be changed by changing
the fork oil amount. Standard spark
plugBR9EVX/NGK
(resistance
type)
Secondary reduction ratio =
Number of rear wheel sprocket
teeth/Number of drive sprocket
teeth
Standard second-
ary reduction ratio3.692 (48/13)
Part name Size Part number
Drive
sprocket
"1"
(STD) 13T 9383B-13218
Rear wheel
sprocket
"2"
47T 1C3-25447-00
(STD) 48T 1C3-25448-00
49T 1C3-25449-00
50T 1C3-25450-00
51T 1C3-25451-00
52T 1C3-25452-00
Standard tire pressure:
100 kPa (1.0 kgf/cm2,
15 psi)
Extent of adjustment:
60–80 kPa (0.6–0.8 kgf/
cm
2, 9.0–12 psi)
Extent of adjustment:
100–120 kPa (1.0–1.2
kgf/cm
2, 15–18 psi)