2008 YAMAHA YZ250F gas type

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Page 24 of 192

YAMAHA YZ250F 2008 Owners Manual 2-2
GENERAL SPECIFICATIONS
Fuel:Type Premium unleaded gasoline only with a research octane
number of 95 or higher.
Tank capacity 7.0 L (1.54 Imp gal, 1.85 US gal)
Carburetor: Type FCR-MX37
Manufactur

Page 30 of 192

YAMAHA YZ250F 2008 Owners Manual 2-8
MAINTENANCE SPECIFICATIONS
CHASSISHeight
240 mm (9.4 in)----
Thickness 22 mm (0.9 in)----
Radiator cap opening pressure 110 kPa (1.1 kg/cm
2, 15.6 psi)----
Radiator capacity (total) 0.58 L (0.51 I

Page 171 of 192

YAMAHA YZ250F 2008 Owners Manual 5-39
REAR SHOCK ABSORBER
HANDLING NOTE
• Support the machine securely so there is no danger of it falling
over.
• This rear shock absorber is pro-
vided with a separate type tank
filled with hig

5-39
REAR SHOCK ABSORBER
HANDLING NOTE
• Support the machine securely so there is no danger of it falling
over.
• This rear shock absorber is pro-
vided with a separate type tank
filled with high-pressure nitro-
gen gas. To prevent the danger
of explosion, read and under-
stand the following information
before handling the shock ab-
sorber. The manufacturer can
not be held responsible for prop-
erty damage or personal injury
that may result from improper
handling.
• Never tamper or attempt to dis-assemble the cylinder or the
tank.
• Never throw the rear shock ab-
sorber into an open flame or
other high heat. The rear shock
absorber may explode as a re-
sult of nitrogen gas expansion
and/ or damage to the hose.
• Be careful not to damage any
part of the gas tank. A damaged
gas tank will impair the damp-
ing performance or cause a
malfunction.
• Take care not to scratch the contact surface of the piston
rod with the cylinder; or oil
could leak out.
• Never attempt to remove the plug at the bottom of the nitro-
gen gas tank. It is very danger-
ous to remove the plug.
• When scrapping the rear shock
absorber, follow the instruc-
tions on disposal.
NOTES ON DISPOSAL (YAMAHA
DEALERS ONLY)
Before disposing the rear shock ab-
sorber, be sure to extract the nitrogen
gas from valve "1". Wear eye protec-
tion to prevent eye damage from es-
caping gas and/or metal chips.
To dispose of a damaged or worn-
out rear shock absorber, take the
unit to your Yamaha dealer for this
disposal procedure.
REMOVING THE BEARING
1. Remove:• Stopper ring (upper bearing) "1"
Press in the bearing while pressing its
outer race and remove the stopper
ring.
2. Remove:• Upper bearing "1"
Remove the bearing by pressing its
outer race.
3. Remove:• Lower bearing "1"
Remove the bearing by pressing its
outer race.
CHECKING THE REAR SHOCK
ABSORBER
1. Inspect:
• Damper rod "1"Bends/damage
→Replace rear
shock absorber assembly.
• Shock absorber "2" Oil leaks
→Replace rear shock
absorber assembly.
Gas leaks
→Replace rear shock
absorber assembly. • Spring "3"
Damage
→Replace spring.
Fatigue
→Replace spring.
Move spring up and down.
• Spring guide "4" Wear/damage
→Replace spring
guide.
• Spring seat "5" Cracks/damage
→Replace.
• Bearing "6"
Free play exists/unsmooth revolu-
tion/rust
→Replace.
INSTALLING THE BEARING
1. Install: • Upper bearing "1"
Install the bearing parallel until the
stopper ring groove appears by
pressing its outer race.
Do not apply the grease on the
bearing outer race because it will
wear the rear shock absorber sur-
face on which the bearing is press
fitted.
2. Install:• Stopper ring (upper bearing) "1"
After installing the stopper ring, push
back the bearing until it contacts the
stopper ring.

Page 181 of 192

YAMAHA YZ250F 2008 Owners Manual 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

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

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