Yamaha YZ125X1 Owner'S Service Manual Download

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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)

<Requirement for selection of sec-
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 PARTS

TIRE 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