air condition YAMAHA YZ250F 2008 Owners Manual

TO THE NEW OWNER
This manual will provide you with a
good basic understanding of fea-
tures, operation, and basic mainte-
nance and inspection items of this
machine. Please read this manual
carefully and completely before oper-
ating your new machine. If you have
any questions regarding the opera-
tion or maintenance of your machine,
please consult your Yamaha dealer.
This manual should be considered a
permanent part of this machine and
should remain with it even if the ma-
chine is subsequently sold.
NOTICE
Some data in this manual may be-
come outdated due to improve-
ments made to this model in the
future. If there is any question you
have regarding this manual or your
machine, please consult your
Yamaha dealer.
F.I.M. MACHINE WEIGHTS
Weights of machines without fuel
The minimum weights for motocross
machines are:
for the class 125 cc: minimum 88 kg
(194 lb)
for the class 250 cc: minimum 98 kg
(216 lb)
for the class 500 cc: minimum 102 kg
(225 lb)
In modifying your machine (e.g., for
weight reduction), take note of the
above limits of weight.
HOW TO USE THIS MANUAL
FINDING THE REQUIRED PAGE
1. This manual consists of seven chapters; "General Information",
"Specifications", "Regular inspec-
tion and adjustments", "Engine",
"Chassis", "Electrical" and "Tun-
ing"
2. The table of contents is at the be-
ginning of the manual. Look over
the general layout of the book be-
fore finding then required chapter and item.
Bend the book at its edge, as
shown, to find the required fore
edge symbol mark and go to a
page for required item and de-
scription.
MANUAL FORMAT
All of the procedures in this manual
are organized in a sequential, step-
by-step format. The information has
been complied to provide the me-
chanic with an easy to read, handy
reference that contains comprehen-
sive explanations of all disassembly,
repair, assembly, and inspection op-
erations.
In this revised format, the condition of
a faulty component will precede an
arrow symbol and the course of ac-
tion required will follow the symbol,
e.g., • Bearings
Pitting/damage
→Replace.
10. PROPERLY SECURE THE
MACHINE BEFORE TRANS-
PORTING IT.
When transporting the ma-
chine in another vehicle, al-
ways be sure it is properly
secured and in an upright po-
sition and that the fuel cock is
in the "OFF" position. Other-
wise, fuel may leak out of the
carburetor or fuel tank.

7-1
ENGINE
TUNING
ENGINE
CARBURETOR SETTING
• The air/fuel mixture will vary de-pending on atmospheric conditions.
Therefore, it is necessary to take
into consideration the air pressure,
ambient temperature, humidity,
etc., when adjusting the carburetor.
• Perform a test run to check for prop- er engine performance (e.g., throt-
tle response) and spark plug(-s)
discoloration or fouling. Use these
readings to determine the best pos-
sible carburetor setting.
It is recommended to keep a record of
all carburetor settings and external
conditions (e.g., atmospheric condi-
tions, track/surface conditions, lap
times) to make future carburetor set-
ting easier.
• The carburetor is a part of the fuel line. Therefore, be sure to in-
stall it in a wellventilated area,
away from flammable objects
and any sources of fire.
• Never look into the carburetor in- take. Flames may shoot out from
the pipe if the engine backfires
while it is being started. Gasoline
may be discharged from the ac-
celerator pump nozzle when the
carburetor has been removed.
• The carburetor is extremely sen-sitive to foreign matter (dirt,
sand, water, etc.). During instal-
lation, do not allow foreign mat-
ter to get into the carburetor.
• Always handle the carburetor and its components carefully.
Even slight scratches, bends or
damage to carburetor parts may
prevent the carburetor from
functioning correctly. Carefully
perform all servicing with the ap-
propriate tools and without ap-
plying excessive force.
• When the engine is stopped or when riding at no load, do not
open and close the throttle un-
necessarily. Otherwise, too
much fuel may be discharged,
starting may become difficult or
the engine may not run well.
• After installing the carburetor,
check that the throttle operates correctly and opens and closes
smoothly.
ATMOSPHERIC CONDITIONS AND
CARBURETOR SETTINGS
The air density (i.e., concentration of
oxygen in the air) determines the rich-
ness or leanness of the air/fuel mix-
ture.
• 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.
EFFECT OF SETTING PARTS IN
RELATION TO THROTTLE VALVE
OPENING
A. Closed
B. Fully open
1. Pilot screw/pilot jet
2. Throttle valve cutaway
3. Jet needle
4. Main jet CONSTRUCTION OF
CARBURETOR AND SETTING
PARTS
The FLATCR carburetor has a prima-
ry main jet. This type of main jet is
perfect for racing machines since it
supplies an even flow of fuel, even at
full load. Use the main jet and the jet
needle to set the carburetor.
The FLATCR carburetor is manufac-
tured with a pilot screw. The pilot
screw adjustment ranges from fully
closed throttle to 1/4 open throttle.
1. Jet needle
2. Pilot air jet
3. Needle jet
4. Main jet
5. Pilot jet
6. Pilot screw
ADJUSTING THE MAIN JET
The richness of the air-fuel mixture at
full throttle can be set by changing the
main jet "1".
If the air-fuel mixture is too rich or too
lean, the engine power will drop, re-
sulting in poor acceleration.
Effects of changing the main jet
(reference)
A. Idle
B. Fully open
1. #180
2. #175
3. #178
Air
tem p. 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
Standard main jet #178
7

7-4
ENGINE
SPECIFICATIONS OF JET NEEDLE
EXAMPLES OF CARBURETOR SETTING DEPENDING ON SYMPTOMDiameter of straight portion
Rich Lean
NPQRS TU
Rich 1 richer NFPN-6 NFPP-6 NFPQ-6 NFPR-6 NFPS-6 NFPT-6 NFPU-6 0.5 richer NFLN-6 NFLP-6 NFLQ-6 NFLR-6 NFLS-6 NFLT-6 NFLU-6STD NFPN-5 NFPP-5 NFPQ-5 NFPR-5 NFPS-5 NFPT-5 NFPU-5
0.5 leaner NFLN-5 NFLP-5 NFLQ-5 NFLR-5 NFLS-5 NFLT-5 NFLU-5
Lean 1 leaner NFPN-4 NFPP-4 NFPQ-4 NFPR-4 NFPS-4 NFPT-4 NFPU-4
Symptom Setting Checking
At full throttle 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
Check that the accelerator pump operates
smoothly.
At full throttle Speed pick-up stops
Slow speed pick-up
Slow response
Sooty spark plug
↓
Rich mixture Decrease main jet calibration no. (Gradual-
ly)
Discoloration of spark plug
→If tan color, it
is in good condition.
If cannot be corrected:
Clogged air filter
Fuel overflow from carburetor
Lean mixture Lower jet needle clip position. (1 groove
down)
The clip position is the jet needle groove on
which the clip is installed.
The positions are numbered from the top.
Check that the accelerator pump operates
smoothly. (except for rich mixture symp-
tom).
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
Poor acceleration Raise jet needle clip position. (1 groove up)
Closed to 1/4 throttle Hard breathing
Speed down Use jet needle with a smaller diameter. Slow-speed-circuit passage
Clogged
→Clean.
Overflow from carburetor
Closed to 1/4 throttle Poor acceleration Use jet needle with a larger diameter.
Raise jet needle clip position. (1 groove up)
Poor response in the low to in-
termediate speeds Raise jet needle clip position.
If this has no effect, lower the jet needle clip
position.
Poor response when throttle is
opened quickly Check overall settings.
Use main jet with a lower calibration no.
Raise jet needle clip position. (1 groove up)
If these have no effect, use a main jet with
a higher calibration no. and lower the jet
needle clip position. Check air filter for fouling.
Check that the accelerator pump operates
smoothly.
Jet needleClip
Groove 7 Groove 6
Groove 5
Groove 4
Groove 3
Groove 2
Groove 1
Leaner
(Standard) Richer

7-6
CHASSIS
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 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.
Adjust the oil amount in 5 cm3 (0.2
Imp oz, 0.2 US oz) increments or
decrements. Too small oil amount
causes the front fork to produce a
noise at full rebound or the rider to
feel some pressure on his hands
or body. Alternatively, too large oil
amount will cause the air spring
characteristics to have a tendency
to be stiffer with the consequent
deteriorated performance and
characteristics. Therefore, adjust
the front fork within the specified
range.
A. Air spring characteristics in relation to oil amount change
B. Load
Secondary reduction ratio =
Number of rear wheel sprocket
teeth/Number of drive sprocket
teeth
Standard secondary
reduction ratio 49/13
(3.769)
Part name Size Part number
Drive
sprocket "1" (STD) 13T 9383B-13218
Rear wheel
sprocket "2" 47T 1C3-25447-00
48T 1C3-25448-00
(STD) 49T 1C3-25449-00 50T 1C3-25450-00
51T 1C3-25451-00
52T 1C3-25452-00
Standard tire pressure: 100 kPa (1.0 kgf/cm
2,
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)
Standard oil amount:330 cm3 (11.6 Imp oz,
11.2 US oz)
Extent of adjustment: 300–375 cm
3(10.6–13.2
Imp oz, 10.1–12.7 US
oz)