2001 YAMAHA YZ426F ad blue

4 - 22
ENG

CAMSHAFTS
6. Remove:
l
Clips
l
Dowel pins
l
Exhaust camshaft
1

l
Intake camshaft
2

NOTE:

Attach a wire
3
to the timing chain to prevent
it from falling into the crankcase.
INSPECTION
Camshaft
1. Inspect:
l
Cam lobes
Pitting/scratches/blue discoloration
®

Replace.
2. Measure:
l
Cam lobes length
a
and
b

Out of specification
®
Replace.
Cam lobes length:
Intake:
a
31.7 ~ 31.8 mm
(1.2480 ~ 1.2520 in)
: 31.6 mm (1.244 in)
b
22.95 ~ 23.05 mm
(0.9035 ~ 0.9075 in)
: 22.85 mm (0.900 in)
Exhaust:
a
31.2 ~ 31.3 mm
(1.2283 ~ 1.2323 in)
: 31.1 mm (1.224 in)
b
22.95 ~ 23.05 mm
(0.9035 ~ 0.9075 in)
: 22.85 mm (0.900 in)
3. Measure:
l
Runout (camshaft)
Out of specification
®
Replace.
Runout (camshaft):
Less than 0.03 mm (0.0012 in)

4 - 34
ENGVALVES AND VALVE SPRINGS
5. Measure:
lRunout (valve stem)
Out of specification ® Replace.
NOTE:
lWhen installing a new valve always replace
the guide.
lIf the valve is removed or replaced always
replace the oil seal.
6. Eliminate:
lCarbon deposits
(from the valve face and valve seat)
7. Inspect:
lValve seats
Pitting/wear ® Reface the valve seat.
Runout limit:
0.01 mm (0.0004 in)
8. Measure:
lValve seat width a
Out of specification ® Reface the valve
seat.
Valve seat width:
Intake:
0.9 ~ 1.1 mm (0.0354 ~ 0.0433 in)
: 1.6 mm (0.0630 in)
Exhaust:
0.9 ~ 1.1 mm (0.0354 ~ 0.0433 in)
: 1.6 mm (0.0630 in)
Measurement steps:
lApply Mechanic’s blueing dye (Dykem) b
to the valve face.
lInstall the valve into the cylinder head.
lPress the valve through the valve guide
and onto the valve seat to make a clear
pattern.
lMeasure the valve seat width. Where the
valve seat and valve face made contact,
blueing will have been removed.
lIf the valve seat is too wide, too narrow, or
the seat is not centered, the valve seat
must be refaced.

4 - 35
ENGVALVES AND VALVE SPRINGS
9. Lap:
lValve face
lValve seat
NOTE:
After refacing the valve seat or replacing the
valve and valve guide, the valve seat and
valve face should be lapped.
Lapping steps:
lApply a coarse lapping compound to the
valve face.
CAUTION:
Do not let the compound enter the gap
between the valve stem and the guide.
lApply molybdenum disulfide oil to the
valve stem.
lInstall the valve into the cylinder head.
lTurn the valve until the valve face and
valve seat are evenly polished, then clean
off all of the compound.
NOTE:
For best lapping results, lightly tap the valve
seat while rotating the valve back and forth
between your hands.
lApply a fine lapping compound to the
valve face and repeat the above steps.
NOTE:
After every lapping operation be sure to
clean off all of the compound from the valve
face and valve seat.
lApply Mechanic’s blueing dye (Dykem) to
the valve face.
lInstall the valve into the cylinder head.
lPress the valve through the valve guide
and onto the valve seat to make a clear
pattern.
lMeasure the valve seat width again. If the
valve seat width is out of specification,
reface and relap the valve seat.

4 - 37
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
Black Black
ab
ASSEMBLY AND INSTALLATION
1. Apply:
lMolybdenum disulfide oil
Onto the valve stem and valve stem
seal.
2. Install:
lValves 1
lValve spring seats 2
lValve stem seals 3
lValve springs 4
lValve spring retainers 5
NOTE:
lMake 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
lInstall the valve springs with the larger pitch
c facing upward.
dSmaller pitch
New

4 - 43
ENG

CYLINDER AND PISTON
Piston pin
1. Inspect:
l
Piston pin
Blue discoloration/grooves
®
Replace,
then inspect the lubrication system.
2. Measure:
l
Piston pin-to-piston clearance
ASSEMBLY AND INSTALLATION
Piston
1. Install:
l
Piston rings
Onto the piston.
NOTE:

l
Be sure to install the piston rings so that the
manufacturer’s marks or numbers are
located on the upper side of the rings.
l
Lubricate the piston and piston rings liberally
with engine oil.
Measurement steps:
l
Measure the outside diameter (piston pin)
a
.
If out of specification, replace the piston
pin.
Outside diameter (piston pin):
18.991 ~ 19.000 mm
(0.7477 ~ 0.7480 in)
l
Measure the inside diameter (piston)
b
.
Inside diameter (piston):
19.004 ~ 19.015 mm
(0.7482 ~ 0.7486 in)
l
Calculate the piston pin-to-piston clear-
ance with the following formula.
Piston pin-to-piston clearance =
Inside diameter (piston)
b
–
Outside diameter (piston pin)
a

l
If out of specification, replace the piston.
Piston pin-to-piston clearance:
0.004 ~ 0.024 mm
(0.00016 ~ 0.00094 in)
: 0.07 mm (0.003 in)

6 - 1
–+ELECELECTRICAL COMPONENTS AND WIRING DIAGRAM
EC600000
ELECTRICAL
EC610000
ELECTRICAL COMPONENTS AND WIRING DIAGRAM
EC611000
ELECTRICAL COMPONENTS
1“ENGINE STOP” button
2TPS (throttle position sensor)
3Neutral switch
4CDI magneto
5Ignition coil
6Spark plug
7CDI unitCOLOR CODE
B...................... Black
Br .................... Brown
G ..................... Green
L ...................... Blue
O ..................... Orange
P...................... Pink
R ..................... Red
EC612000
WIRING DIAGRAM
7
15
6
2 3 4
LY
BYL
B/LL
G
P
W
R
Y
B
G
BR PBrWG
RB Br WP
R/W WL/W SbW R/W Sb L/WY
B/LL Sb B OY
LB/L B OSbSb
L/WR/W
WSb
B/L
B/WB/W
B/WB/W
Br
BB
LBBY
O
Sb .................... Sky blue
W ..................... White
Y ...................... Yellow
B/L ................... Black/Blue
B/W .................. Black/White
L/W .................. Blue/White
R/W .................. Red/White




6

6 - 6
–+ELECIGNITION SYSTEM
2. Inspect:
lSource coil 1 resistance
Out of specification ® Replace.
3. Inspect:
lSource coil 2 resistance
Out of specification ® Replace. Tester (+) lead ® Brown lead 1
Tester (–) lead ® Green lead 2
Source coil 1
resistanceTester selector
position
720 ~ 1,080 W
at 20 ˚C (68 ˚F)W ´ 100
Tester (+) lead ® Pink lead 1
Tester (–) lead ® Black lead 2
Source coil 2
resistanceTester selector
position
44 ~ 66 W at
20 ˚C (68 ˚F)W ´ 10
NEUTRAL SWITCH INSPECTION
1. Inspect:
lNeutral switch conduct
No continuity while in neutral ® Replace.
Continuity while in gear ® Replace.
EC628000
CDI UNIT INSPECTION
Check all electrical components. If no fault is
found, replace the CDI unit. Then check the
electrical components again.Tester (+) lead ® Sky blue lead 1
Tester (–) lead ® Ground
Sb
1GroundTester selec-
tor position
NEUTRALW ´ 1IN GEAR
–+ W´1
R/W
W Sb L/W
1

6 - 8
–+ELEC

TPS (THROTTLE POSITION SENSOR) SYSTEM
EC624000
COUPLERS AND LEADS CONNECTION
INSPECTION
1. Check:
l
Couplers and leads connection
Rust/dust/looseness/short-circuit
®

Repair or replace.
TPS COIL INSPECTION
1. Inspect:
l
TPS coil resistance
Out of specification
®
Replace.
2. Loosen:
l
Throttle stop screw
NOTE:

Turn out the throttle stop screw until the throt-
tle shaft is in the full close position.
Tester (+) lead
®
Blue lead
1

Tester (–) lead
®
Black lead
2

TPS coil
resistanceTester selector
position
4 ~ 6 k
W
at
20 ˚C (68 ˚F)k
W

´
1
3. Inspect:
l
TPS coil variable resistance
Check that the resistance in increased
as the throttle grip is moved from the
full close position to the full open posi-
tion.
Out of specification
®
Replace.
Tester (+) lead
®
Yellow lead
1

Tester (–) lead
®
Black lead
2

TPS coil variable
resistance
Tester selec-
tor position
Full closedFull opened
k
W

´
1 0 ~ 2 kW at
20 ˚C
(68 ˚F)4 ~ 6 kW at
20 ˚C
(68 ˚F)