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 SYMPTOMGeneral 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 richer6BFY44-72-3 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 leaner6BFY44-72-2 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 mixtureIncrease 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 mixtureDecrease main jet calibration no. (Gradual-
ly)
*In case of racing slight 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
7-6
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)
