2003 YAMAHA YZ450F engine

GEN
INFO
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DECOMPRESSION LEVER
DECOMPRESSION LEVER
An auto decompression device is standard equipment on the machine. There is no need to operate
the decompression lever in normal use because the auto decompression device is working.
Use the decompression lever when the engine fails to start or when you want to step down the kick
starter with a lighter force.
For engine starting failure, refer to “STARTING AND BREAK-IN”.
Installation (Supplied for Europe and optional for other destinations)
For installation, follow the steps below.
Order Part name Q’ty Remarks
1 Oil seal 1
2 Torsion spring 1
3 Decompression shaft 1
4 Decompression cable guide 1
5 Flange bolt 1 M 6 × 1.0 [L=14 mm (0.55 in)]
6 Decompression cable 1
7 Decompression lever 1
8 Boot 1
9 Decompression lever holder (upper) 1
10 Collar 1
11 Panhead screw 1 M 5 × 0.8 [L=20 mm (0.79 in)]
12 Nut 1 M 5 × 0.8
13 Decompression lever holder (lower) 1
14 Hexagon bolt 2 M 5 × 0.8 [L=16 mm (0.63 in)]
14
13
11
10 8
7
9
12 6
5
4
1
2
3
T R..10 Nm (1.0 m •
kg, 7.2 ft • Ib)
T R..7 Nm (0.7 m •
kg, 5.1 ft • Ib)
T R..4 Nm (0.4 m •
kg, 2.9 ft • Ib)
B
B
B
B
BE

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GEN
INFO
DECOMPRESSION LEVER
INSTALLATION OF DECOMPRESSION
LEVER
Preparations
1. Remove the seat.
2. Drain the coolant.
Refer to “COOLANT REPLACEMENT”
section in the CHAPTER 3.
3. Remove:
Radiator
Refer to “RADIATOR” section in the
CHAPTER 4.
Cylinder head cover
Refer to “CAMSHAFTS” section in the
CHAPTER 4.
Number plate
4. Remove:
Plug 1
Bolt 2
Plain washer 3
NOTE:
Make a hole in the resin side a and remove
the plug.
Installation
1. Install:
Oil seal 1
NOTE:
Install the oil seal, as shown, where the plug
was removed.
Apply the lithium soap base grease on the oil
seal lip.
2. Install:
Torsion spring 1
Decompression shaft 2
Plain washer 3
Bolt (decompression shaft) 4
Decompression cable guide 5
Bolt (decompression cable guide) 6
Decompression cable 7
NOTE:
When installing the decompression shaft,
install the torsion spring in the location as
shown.
For the decompression shaft securing bolt
and plain washer, use the ones removed
from the same machine.
Apply the engine oil on the decompression
shaft.
23
1 a
1B
T R..7 Nm (0.7 m · kg, 5.1 ft · lb)
T R..10 Nm (1.0 m · kg, 7.2 ft · lb)

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GEN
INFO
DECOMPRESSION LEVER ADJUSTMENT
1. Check:
Decompression lever free play
2. Adjust:
Decompression lever free play
FOR PROPER USE
CAUTION:
Never use the decompression lever after
the engine is started. The engine may be
damaged if you use the decompression
lever while it is running.
NOTE:
Refer to “STARTING AND BREAK-IN” for
starting except when “The engine fails to start.”
The engine fails to start
Pull the decompression lever to the handlebar,
and while holding the lever, kick the kick
starter 10 to 20 times to clear the engine of the
rich mixture.Checking steps:
Remove the timing plug.
Turn the crankshaft counterclockwise to
align the T.D.C. mark a on the rotor with
the align mark b on the crankcase cover
when piston is at T.D.C. on compression
stroke.
Check the free play c.
Free play:
3 ~ 5 mm (0.12 ~ 0.20 in)
Adjustment steps:
Loosen the locknut 1.
Turn the adjuster 2 in or out until the
adjustment is suitable.
Tighten the locknut.
CAUTION:
After the adjustment, start the engine
and make sure that the engine does not
stop when the handlebars are turned
fully both ways. If the decompression
cable is too tense, the engine will stop by
the decompression system.
2 1 c
DECOMPRESSION LEVER

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GEN
INFO
FUEL
Always use the recommended fuel as stated
below. Also, be sure to use new gasoline the
day of a race.
CAUTION:
Use only unleaded gasoline. The use of
leaded gasoline will cause severe damage
to the engine internal parts such as valves,
piston rings, and exhaust system, etc.
NOTE:
If knocking or pinging occurs, use a different
brand of gasoline or higher octane grade.
WARNING
For refueling, be sure to stop the engine
and use enough care not to spill any fuel.
Also be sure to avoid refueling close to a
fire.
Refuel after the engine, exhaust pipe, etc.
have cooled off.
Recommended fuel:
Except for ZA:
Premium unleaded gasoline
only with a research octane
number of 95 or higher.
For ZA:
Premium gasoline
FUEL

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GEN
INFO
STARTING AND BREAK-IN
STARTING AND BREAK-IN
WARNING
Never start or run the engine in a closed
area. The exhaust fumes are poisonous;
they can cause loss of consciousness and
death in a very short time. Always operate
the machine in a well-ventilated area.
CAUTION:
The carburetor on this motorcycle has a
built-in accelerator pump. Therefore,
when starting the engine, do not operate
the throttle or the spark plug will foul.
Unlike a two-stroke engine, this engine
cannot be kick started when the throttle is
open because the kick starter may kick
back. Also, if the throttle is open the air/
fuel mixture may be too lean for the
engine to start.
Before starting the machine, perform the
checks in the pre-operation check list.
STARTING A COLD ENGINE
1. Inspect the coolant level.
2. Turn the fuel cock to “ON”.
3. Shift the transmission into neutral.
4. Fully open the cold starter knob 1.
5. Kick the kick starter.
WARNING
Do not open the throttle while kicking the
kick starter. Otherwise, the kick starter may
kick back.

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GEN
INFO
STARTING AND BREAK-IN
6. Return the cold starter knob to its original
position and run the engine at 3,000 ~
5,000 r/min for 1 or 2 minutes.
NOTE:
Since this model is equipped with an accelera-
tor pump, if the engine is raced (the throttle
opened and closed), the air/fuel mixture will be
too rich and the engine may stall. Also unlike a
two-stroke engine, this model can idle.
CAUTION:
Do not warm up the engine for extended
periods of time.

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GEN
INFO
STARTING AND BREAK-IN
STARTING A WARM ENGINE
Do not operate the cold starter knob and throt-
tle. Pull the hot starter lever 1 and start the
engine by kicking the kick starter forcefully with
a firm stroke.
As soon as the engine starts, release the hot
starter lever to close the air passage.
Restarting an engine after a fall
Pull the hot starter lever and start the engine.
As soon as the engine starts, release the hot
starter lever to close the air passage.
The engine fails to start
Pull the hot starter lever all the way out and
while holding the lever, kick the kick starter 10
to 20 times to clear the engine.
Then, restart the engine.
Refer to “Restarting an engine after a fall”.
* Operate the throttle grip before kick starting.
CAUTION:
Observe the following break-in procedures
during initial operation to ensure optimum
performance and avoid engine damage.
Throttle
grip oper-
ation*Cold
starter
knobHot
starter
lever
Starting a cold engine
Air temperature = less than
5 ˚C (41 ˚F)Open 3
or 4 timesON OFF
Air temperature = more
than 5 ˚C (41 ˚F)None ON OFF
Air temperature (normal tem-
perature) = between 5 ˚C
(41 ˚F) and 25 ˚C (77 ˚F)None ON/OFF OFF
Air temperature = more
than 25 ˚C (77 ˚F)None OFF OFF
Starting an engine after a long
period of timeNone ON OFF
Restarting a warm engine None OFF ON
Restarting an engine after a fall None OFF ON

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GEN
INFO
BREAK-IN PROCEDURES
1. Before starting the engine, fill the fuel
tank with the fuel.
2. Perform the pre-operation checks on the
machine.
3. Start and warm up the engine. Check the
idle speed, and check the operation of
the controls and the “ENGINE STOP”
button. Then, restart the engine and
check its operation within no more than 5
minutes after it is restarted.
4. Operate the machine in the lower gears
at moderate throttle openings for five to
eight minutes.
5. Check how the engine runs when the
motorcycle is ridden with the throttle 1/4
to 1/2 open (low to medium speed) for
about one hour.
6. Restart the engine and check the opera-
tion of the machine throughout its entire
operating range. Restart the machine
and operate it for about 10 to 15 more
minutes. The machine will now be ready
to race.
CAUTION:
After the break-in or before each race,
you must check the entire machine for
loose fittings and fasteners as per
“TORQUE-CHECK POINTS”.
Tighten all such fasteners as required.
When any of the following parts have
been replaced, they must be broken in.
CYLINDER AND CRANKSHAFT:
About one hour of break-in operation is
necessary.
PISTON, RING, VALVES, CAMSHAFTS
AND GEARS:
These parts require about 30 minutes of
break-in operation at half-throttle or less.
Observe the condition of the engine care-
fully during operation.
STARTING AND BREAK-IN