fuel YAMAHA YZ85 2003 Owner's Guide

7 - 1
TUN
EC700000
TUNING
EC710000
ENGINE
Carburetor setting

The role of fuel is to cool the engine, and in
the case of a 2-stroke engine, to lubricate the
engine in addition to power generation.
Accordingly, if a mixture of air and fuel is too
lean, abnormal combustion will occur, and
engine seizure may result. If the mixture is
too rich, spark plugs will get wet with oil, thus
making it impossible to bring the engine into
full play or if the worst comes to the worst,
the engine may stall.

The richness of the air-fuel mixture required
for the engine will vary with atmospheric con-
ditions of the day and therefore, the settings
of the carburetor must be properly suited to
the atmospheric conditions (air pressure,
humidity and temperature).

Finally, the rider himself must make a test
run and check his machine for conditions
(pick-up of engine speed, road surface con-
ditions) and for the discoloration of the spark
plug(s).
After taking these into consideration, he must
select the best possible carburetor settings.
* It is advisable to make a note of settings,
atmospheric conditions, road surface condi-
tion, lap-time, etc. so that the memorandum
can be used as a reference useful for future.
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.
Air temp. HumidityAir
pressure
(altitude)Mixture Setting
High High Low (high) Richer Leaner
Low Low High (low) Leaner Richer
SETTING

7 - 3
TUNSETTING
Effects of the setting parts on the throttle
valve opening
1Throttle valve opening
2Full-open
3Full-closed
Setting partThrottle valve openingFull-closed1/4 1/2 3/4 Full-open
Pilot jet
Pilot air screw
Jet needle
Diameter of
straight por-
tion
Clip position
Main jet
5PA70040
Main jet adjustment
The richness of air-fuel mixture with 3/4 ~ 4/4
throttle can be set by changing the main jet 1
.
1. Spark plug is too hot.
Select a main jet having higher calibrating
No. than standard. (To be enriched)
2. Spark plug is wet.
Select a main jet having lower calibrating
No. than standard. (To be leaned out)Standard main jet #138
5PA70050
Pilot air screw adjustment
The richness of the air-fuel mixture with the
throttle fully closed to 1/4 open can be set by
turning the pilot air screw 1
.
Turning in the pilot air screw will make the mix-
ture enrich at low speeds, and turning it out will
lean it.
Standard pilot air
screw position2 turns out
(example)
5PA70060

7 - 4
TUNSETTING
Pilot jet adjustment
The richness of air-fuel mixture with the throttle
fully closed to 1/2 open can be set by turning
the pilot jet 1. It is changed when adjustment
cannot be made by the pilot air screw alone.
A larger size jet results in a richer mixture at
low speeds, and a smaller size in a leaner mix-
ture.
Standard pilot jet #45
5PA70070
Jet needle adjustment
The richness of air-fuel mixture with 1/4 ~ 3/4
throttle can be set by changing the jet needle
1.
A smaller diameter jet needle results in a richer
mixture at middle speeds, and a larger diame-
ter in a leaner mixture.
Standard jet needle NBKF
5PA70080

7 - 6
TUNSETTING
EC71Q000
Carburetor setting parts
Part name Size Part number
Main jet
1Rich
(STD)
Lean#148
#145
#142
#140
#138
#135
#1324MX-14943-87
4MX-14943-36
4MX-14943-86
4MX-14943-35
4MX-14943-85
4MX-14943-34
4MX-14943-84
Pilot jet
2Rich
(STD)
Lean#48
#45
#424MX-14948-06
4MX-14948-05
4MX-14948-04
Jet needle
3
Refer to the following
table about the
change in the air-fuel
mixture ratio due to a
different jet needle.
NBLE
NBLF
NBLG
NBKE
NBKF
NBKG5PA-14916-LE
5PA-14916-LF
5PA-14916-LG
5PA-14916-KE
5PA-14916-KF
5PA-14916-KG
Diameter of straight portion
Clip positionRich Lean
φ 2.405 mm (0.0947 in)φ 2.415 mm (0.0951 in)φ 2.425 mm (0.0955 in)
Rich
Lean1 richer NBKE-3rd groove NBKF-3rd groove NBKG-3rd groove
0.5 richer NBLE-2nd groove NBLF-2nd groove NBLG-2nd groove
STD NBKE-2nd groove NBKF-2nd groove (STD) NBKG-2nd groove
0.5 leaner NBLE-1st groove NBLF-1st groove NBLG-1st groove
1 leaner NBKE-1st groove NBKF-1st groove NBKG-1st groove
5PA70100

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