light YAMAHA YZ85 2003 User Guide

4 - 12
ENGCARBURETOR AND REED VALVE
Carburetor
1. Install:

Pilot air screw 1

Throttle stop screw 2
Note the following installation points:

Turn in the pilot air screw until it is lightly
seated.

Turn out the pilot air screw by the number
of turns recorded before removing.
Pilot air screw:
2 turns out (example)
5PA40260
2. Install:

Starter plunger 1
5PA40270
3. Install:

Pilot jet 1

Main nozzle 2

Main nozzle holder 3

Main jet 4
NOTE:
Install the main nozzle with its chamfered side
a facing the carburetor.
5PA40280
4. Install:

Needle valve 1

Float 2

Float pin 3
NOTE:

After installing the needle valve to the float,
install them to the carburetor.

Check the float for smooth movement.
5PA40290

4 - 54
ENG
REMOVAL POINTS
Crankcase
1. Remove:

Right crankcase 1
Use the crankcase separating tool 2.
NOTE:

Fully tighten the tool holding bolts, but make
sure the tool body is parallel with the case. If
necessary, one screw may be backed out
slightly to level tool body.

As pressure is applied, alternately tap on the
front engine mounting boss and transmission
shafts.
CAUTION:

Turn the segment 3 to the position
shown in the figure so that it does not
contact the crankcase.

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.
EC4N3301
Crankshaft
1. Remove:

Crankshaft 1
Use the crankcase separating tool 2.
NOTE:
Make appropriate bolts 3 as shown available
by yourself and attach the tool with them.
CAUTION:
Do not use a hammer to drive out the
crankshaft.
Crankcase separating tool:
YU-1135-A/90890-01135
Crankcase separating tool:
YU-1135-A/90890-01135
5PA41600
5PA41610
5PA41620
CRANKCASE AND CRANKSHAFT

4 - 58
ENG
4. Install:

Dowel pin 1

Right crankcase 2
To left crankcase 3.
NOTE:

Turn the shift cam 4 to the position shown in
the figure so that it does not contact the
crankcase when installing the crankcase.

Fit the right crankcase onto the left crank-
case. 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).
5PA41760
5PA41770
5. Install:

Clamp 1

Screw (crankcase) 2
NOTE:
Tighten the crankcase tightening screws in
stage, using a crisscross pattern.
5PA41780
T R..8 Nm (0.8 m · kg, 5.8 ft · lb)
6. Install:

Crankcase oil seal holder 1

Bolt (crankcase oil seal holder) 2
7. Remove:

Sealant
Forced out on the cylinder mating surface.
8. Apply:

Engine oil
To the crank pin, bearing, oil delivery hole
and connecting rod end washer.
9. Check:

Crankshaft and transmission operation
Unsmooth operation → Repair.
5PAR0010
T R..20 Nm (2.0 m · kg, 14 ft · lb)
CRANKCASE AND CRANKSHAFT

4 - 60
ENG
EC4H3000
REMOVAL POINTS
Transmission
1. Remove:

Main axle 1

Drive axle 2
NOTE:

Remove assembly carefully. Note the posi-
tion of each part. Pay particular attention to
the location and direction of shift forks.

Remove the main axle and drive axle by tap-
ping lightly on the transmission drive axle
with a soft hammer.
5PA41800
EC4H4000
INSPECTION
EC4H4200
Gears
1. Inspect:

Matching dog a

Gear teeth b

Shift fork groove c
Wear/damage → Replace.
5PA41810
2. Check:

Gears movement
Unsmooth movement → Disassemble, and
then repair or replace.
5PA41820
EC4H4600
Bearing
1. Inspect:

Bearing 1
Rotate inner race with a finger.
Rough spot/seizure → Replace.
5PA41830
TRANSMISSION, SHIFT CAM AND SHIFT FORK

5 - 22
CHAS
Front brake hose
1. Install:

Copper washer 1

Brake hose 2

Union bolt 3
WARNING
Always use new copper washers.
CAUTION:
Install the brake hose so that its pipe por-
tion a directs as shown and lightly
touches the projection b on the brake
master cylinder.
5PA50660
T R..30 Nm (3.0 m · kg, 22 ft · lb)
5PA50670
2. Install:

Protector 1

Bolt (protector) 2
NOTE:
Fit the brake hose into the guides a of the pro-
tector.
5PA50680
T R..7 Nm (0.7 m · kg, 5.1 ft · lb)
3. Install:

Brake hose holder 1

Bolt (brake hose holder) 2
NOTE:

Align the brake hose holder with the projec-
tion a on the front fork and clamp the brake
hose.

Pass the brake hose behind the rib b on the
front fork.
5PA50690
T R..10 Nm (1.0 m · kg, 7.2 ft · lb)
FRONT BRAKE AND REAR BRAKE

5 - 23
CHAS
4. Install:

Copper washer 1

Brake hose 2

Union bolt 3
WARNING
Always use new copper washers.
CAUTION:
Install the brake hose so that its pipe por-
tion a lightly touches the projection b on
the brake caliper.
5PA50700
5PA50710
T R..26 Nm (2.6 m · kg, 19 ft · lb)
Rear brake hose
1. Install:

Copper washer 1

Brake hose 2

Union bolt 3
WARNING
Always use new copper washers.
CAUTION:
Install the brake hose so that its pipe por-
tion a directs as shown and lightly
touches the projection b on the brake
master cylinder.
5PA50720
5PA50730
T R..26 Nm (2.6 m · kg, 19 ft · lb)
FRONT BRAKE AND REAR BRAKE

5 - 24
CHAS
2. Install:

Copper washer 1

Brake hose 2

Union bolt 3
WARNING
Always use new copper washers.
CAUTION:
Install the brake hose so that its pipe por-
tion a lightly touches the projection b on
the brake caliper.
5PA50740
5PA50750
T R..26 Nm (2.6 m · kg, 19 ft · lb)
3. Install:

Brake hose holder 1

Screw (brake hose holder) 2
5PA50760
T R..1 Nm (0.1 m · kg, 0.7 ft · lb)
FRONT BRAKE AND REAR BRAKE

7 - 7
TUN
Road condition and examples of carburetor setting
NOTE:
Optimum pilot air screw setting can be obtained by adding the ex-factory number of the same screw
back-out turns to any required value provided in the chart.
For example, if the ex-factory number is “2”, add “2” to the value chosen in the chart.
Examples of carburetor setting depending on symptomConditions
PartsGeneral condition Sandy condition
Under 10 °C
(50 °F)
(Winter)15 ~ 25 °C
(59 ~ 77 °F)
(Spring, Autumn)
Over 30 °C
(86 °F)
(Summer)Under 10 °C
(50 °F)
(Winter)15 ~ 25 °C
(59 ~ 77 °F)
(Spring, Autumn)
Over 30 °C
(86 °F)
(Summer)
Main jet #140 #138 #135 ~ #138 #142 #142 #142
Jet needle NBKF-2 NBKF-2 NBKF-2 NBKF-2 NBKF-2 NBKF-2
Pilot jet #45 #45 #45 #48 #48 #48
Pilot air screw Zero Zero +1/4–1/2–1/2 Zero ~ +1/2
Symptom Setting Checking
At full throttle
Stall at high speeds
*Hard breathing
Shearing noise
Whitish spark plug
Lean mixtureIncrease main jet calibration no.
(Gradually)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
At full throttle
Speed pick-up stops
Slow speed pick-up
Slow response
Sooty spark plug
Rich mixtureDecrease main jet calibration no.
(Gradually)
*In case of racing slight enrich-
ment of mixture reduces engine
trouble.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 Use jet needle with a smaller
diameter, or NBLF.
Lower jet needle clip position.
The clip position is the jet needle
groove on which the clip is
installed. The positions are num-
bered from the top. Rich mixture Use jet needle with a larger diam-
eter, or NBKG.
Raise jet needle clip position.
1/4 ~ 3/4 throttle
*Hard breathing
Lack of speedUse jet needle with a smaller
diameter, or NBLF.
Lower jet needle clip position.
1/4 ~ 1/2 throttle
Slow speed pick-up
White smoke
Poor accelerationUse jet needle with a larger diam-
eter, or NBKG.
Raise jet needle clip position.
Closed to 1/4 throttle
*Hard breathing
Speed downUse jet needle with a smaller
diameter.
Turn out pilot air screw.
Leaner
↑(Standard)
↓
Richer
SETTING

7 - 9
TUNSETTING
Change of the heat range of spark plugs
Judging from the discoloration of spark plugs,
if they are found improper, it can be corrected
by the following two methods; changing carbu-
retor settings and changing the heat range of
spark plug.

In principle, it is advisable to first use spark
plugs of standard heat range, and judging
from the discoloration of spark plugs, adjust
carburetor settings.

If the calibration No. of the main jet must be
changed by ±30, it is advisable to change the
heat range of spark plugs and newly select
the proper main jet.
NOTE:

When checking the discoloration of spark
plugs, be sure to stop the engine immedi-
ately after a run and check.

Avoid racing.

When changing the heat range of spark
plugs, never attempt to change it more than
±1 rank.

When using a spark plug other than stan-
dard, check its heat range against the stan-
dard and check that it is a resistance type.

Note that even if the discoloration seems
proper, it may slightly vary with the spark
plug maker and oil in use.Standard spark plugBR10EG/NGK
(resistance type)
5PA70110