2002 YAMAHA YZ426F light

4 - 38
ENGVALVES AND VALVE SPRINGS
Combination of cylinder head and valve
lifter
1. Combination:
For this combination, match the paint
color on the cylinder head with that on
the valve lifter according to the chart
below.
NOTE:
When you purchase a cylinder head, you can-
not designate its size. Choose the valve lifter
that matches the above chart.Combination
Cylinder head mark
a (color)Valve lifter mark
b (color)
Blue Blue
Yellow Yellow
Purple Black
ab
ASSEMBLY AND INSTALLATION
1. Apply:
Molybdenum disulfide oil
Onto the valve stem and valve stem
seal.
2. Install:
Valves 1
Valve spring seats 2
Valve stem seals 3
Valve springs 4
Valve spring retainers 5
NOTE:
Make sure that each valve is installed in its
original place, also referring to the painted
color as follows.
Intake (middle) a: light blue
Intake (right/left) b: white
Exhaust: not paint
Install the valve springs with the larger pitch
c facing upward.
dSmaller pitch
New

4 - 39
ENGVALVES AND VALVE SPRINGS
3. Install:
Valve cotters
NOTE:
While compressing the valve spring with a
valve spring compressor 1 install the valve
cotters.
4. To secure the valve cotters onto the
valve stem, lightly tap the valve tip with a
piece of wood.
CAUTION:
Hitting the valve tip with excessive force
could damage the valve.
Valve spring compressor:
YM-4019/90890-04019
5. Install:
Adjusting pad 1
Valve lifter 2
NOTE:
Apply the engine oil on the valve lifters.
Apply the molybdenum disulfide oil on the
valve stem end.
Valve lifter must turn smoothly when rotated
with a finger.
Be careful to reinstall valve lifters and pads
in their original place.

4 - 86
ENGCRANKCASE AND CRANKSHAFT
REMOVAL POINTS
Crankcase
1. Separate:
Crankcase (right)
Crankcase (left)
Separation steps:
Remove the crankcase bolts, hose guide
and clutch cable holder.
NOTE:
Loosen each bolt 1/4 of a turn at a time and
after all the bolts are loosened, remove
them.
Remove the crankcase (right).
NOTE:
Place the crankcase with its left side
downward and split it by inserting a screw-
driver tip into the splitting slit a in the
crankcase.
Lift the crankcase (right) horizontally while
lightly patting the case splitting slit and
engine mounting boss using a soft ham-
mer, and leave the crankshaft and trans-
mission with the crankcase (left).
CAUTION:
Use soft hammer to tap on the case half.
Tap only on reinforced portions of case.
Do not tap on gasket mating surface.
Work slowly and carefully. Make sure the
case halves separate evenly. If one end
“hangs up”, take pressure off the push
screw, realign, and start over. If the
cases do not separate, check for a
remaining case screw or fitting. Do not
force.
Remove the dowel pins and O-ring.
a

4 - 91
ENGCRANKCASE AND CRANKSHAFT
5. Install:
Dowel pin 1
O-ring 2
Crankcase (right)
To crankcase (left).
NOTE:
Fit the crankcase (right) onto the crankcase
(left). Tap lightly on the case with soft ham-
mer.
When installing the crankcase, the connect-
ing rod should be positioned at TDC (top
dead center).
6. Tighten:
Hose guide 1
Clutch cable holder 2
Bolt (crankcase)
NOTE:
Tighten the crankcase tightening bolts in
stage, using a crisscross pattern.
7. Install:
Oil delivery pipe
O-ring
Bolt (oil delivery pipe)
8. Install:
Timing chain
Timing chain guide (rear)
Bolt (timing chain guide)
9. Remove:
Sealant
Forced out on the cylinder mating sur-
face.
10. Apply:
Engine oil
To the crank pin, bearing and oil deliv-
ery hole.
11. Check:
Crankshaft and transmission operation.
Unsmooth operation → Repair.
New
T R..12 Nm (1.2 m · kg, 8.7 ft · lb)
New
T R..10 Nm (1.0 m · kg, 7.2 ft · lb)
T R..10 Nm (1.0 m · kg, 7.2 ft · lb)

4 - 93
ENGTRANSMISSION, SHIFT CAM AND SHIFT FORK
EC4H3000
REMOVAL POINTS
Shift fork, shaft cam and transmission
1. Remove:
Shift forks
Shift cam
Main axle
Drive axle
NOTE:
Tap lightly on the transmission drive axle and
shift cam with a soft hammer to remove.
Remove assembly carefully. Note the posi-
tion of each part. Pay particular attention to
the location and direction of shift forks.
EC4H4000
INSPECTION
EC4H4200
Gears
1. Inspect:
Matching dog a
Gear teeth b
Shift fork groove c
Wear/damage → Replace.
2. Inspect:
O-ring 1
Damage → Replace.
3. Check:
Gears movement
Unsmooth movement → Repair or
replace.
EC4H4600
Bearing
1. Inspect:
Bearing 1
Rotate inner race with a finger.
Rough spot/seizure → Replace.

5 - 18
CHASFRONT BRAKE AND REAR BRAKE
2. Install:
Copper washer 1
Union bolt 2
WARNING
Always use new copper washers.
ACHTUNG:CAUTION:
Install the brake hose so that its pipe por-
tion a directs as show and lightly touches
the projection b on the caliper.
New
T R..30 Nm (3.0 m · kg, 22 ft · lb)
3. Install:
Brake hose holder 1
Caliper 2
Bolt (caliper) 3
NOTE:
Fit the brake hose holder cut a over the pro-
jection b on the front fork and clamp the brake
hose.
4. Tighten:
Pad pin 4
5. Install:
Pad pin plug 5
EC5A5100
Rear caliper
1. Install:
Pad support 1
Brake pad 2
Pad pin 3
NOTE:
Install the brake pads with their projections
a into the caliper recesses b.
Temporarily tighten the pad pin at this point.
T R..23 Nm (2.3 m · kg, 17 ft · lb)
T R..18 Nm (1.8 m · kg, 13 ft · lb)
T R..3 Nm (0.3 m · kg, 2.2 ft · lb)

5 - 24
CHASFRONT BRAKE AND REAR BRAKE
4. Install:
Copper washer 1
Brake hose 2
Union bolt 3
WARNING
Always use new copper washers.
ACHTUNG:CAUTION:
Install the brake hose so that its pipe por-
tion a directs as show and lightly touches
the projection b on the master cylinder.
New
T R..30 Nm (3.0 m · kg, 22 ft · lb)
EC5A5500
Rear brake hose
1. Install:
Copper washer 1
Brake hose 2
Copper washer 3
Union bolt 4
WARNING
Always use new copper washers.
ACHTUNG:CAUTION:
When installing the brake hose, first align
the tooth a on the copper washer 1 with
the projection b on the master cylinder.
Then, align the pipe portion c on the end
of the brake hose with the tooth d on the
copper washer 1 and tighten the union
bolt.
New
New
T R..30 Nm (3.0 m · kg, 22 ft · lb)

7 - 2
TUN
CAUTION:


The carburetor is extremely sensitive to
foreign matter (dirt, sand, water, etc.).
During installation, do not allow foreign
matter to get into the carburetor.

Always handle the carburetor and its
components carefully. Even slight
scratches, bends or damage to carbure-
tor parts may prevent the carburetor from
functioning correctly. Carefully perform
all servicing with the appropriate tools
and without applying excessive force.

When the engine is stopped or when
riding at no load, do not open and close
the throttle unnecessarily. 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 richness or lean-
ness of the air/fuel mixture. Therefore, refer
to the above table for mixture settings.
That is:

Higher temperature expands the air with its
resultant reduced density.

Higher humidity reduces the amount of oxy-
gen in the air by so much of the water vapor
in the same air.

Lower atmospheric pressure (at a high alti-
tude) reduces the density of the air.
Air temp. HumidityAir
pressure
(altitude)Mixture Setting
High High Low (high) Richer Leaner
Low Low High (low) Leaner Richer
SETTING