check engine YAMAHA YZ85 2005 Owner's Manual

6 - 3
–+ELECIGNITION SYSTEM
SPARK GAP TEST
1. Disconnect the spark plug cap from spark
plug.
2. Connect the dynamic spark tester 1 (igni-
tion checker 2) as shown.

Spark plug cap 3

Spark plug 4
ÈFor USA and CDN
ÉExcept for USA and CDN
3. Kick the kickstarter crank.
4. Check the ignition spark gap.
5. Start engine, and increase spark gap until
misfire occurs. (for USA and CDN only)
Minimum spark gap:
6.0 mm (0.24 in)
È
5PA60030
É
5PA60040
EC624000
COUPLERS AND LEADS CONNECTION
INSPECTION
1. Check:

Couplers and leads connection
Rust/dust/looseness/short-circuit → Repair
or replace.
ENGINE STOP SWITCH INSPECTION
1. Inspect:

Engine stop switch conduct
No continuous while being pushed → Replace.
Continuous while being freed → Replace. Tester (+) lead
→ Black/White lead
1
Tester (–) lead
→ Black lead
2
B/W
1B
2Tester selec-
tor position
PUSH IN
Ω
× 1
FREE
5PA60050

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 - 2
TUN
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.
Test run
After warming up the engine equipped with the
standard type carburetor(s) and spark plug(s),
run two or three laps of the circuit and check
the smooth operation of the engine and discol-
oration of spark plug(s).
ÈNormal
ÉOver burned (too lean)
ÊOil fouled (too rich)
Discoloration Condition of spark plug
NormalInsulator is dry and burnt
brown.
Over burned
(too lean)Insulator is whitish.
Oil fouled
(too rich)Insulator is sooty and wet.
5PA70010
È
É
Ê
SETTING

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 - 8
TUN
* marked: In case of hard breathing, check the carburetor breather hoses for clogging. Closed to 1/4 throttle
Poor acceleration
White smokeUse jet needle with a larger diam-
eter.
Turn in pilot air screw.
Unstable at low speeds
Pinking noiseLower jet needle clip position.
(1 groove down)
Turn in pilot air screw.
Poor response at
extremely low speedReduce pilot jet calibration No.
Turn out pilot air screw.
If not effect, reverse the above
procedures.Dragging brake
Overflow from carburetor
Poor response in the low
to intermediate speedsRaise jet needle clip position.
If this has no effect, lower the jet
needle clip position.
Poor response when
throttle is opened quicklyCheck overall settings.
Use main jet with a lower calibra-
tion no.
Raise jet needle clip position.
Use jet needle with a larger diam-
eter.
If these have no effect, use a main
jet with a higher calibration no.
and lower the jet needle clip posi-
tion.Check air filter for fouling.
Poor engine operation Turn in pilot air screw.
Adjust the throttle stop screw.Check throttle valve operation. Symptom Setting Checking
This should be taken simply for an example. It
is necessary to set the carburetor while check-
ing the operating conditions of the engine and
discoloration of spark plugs.
Normally, carburetor setting is made by means
of the main jet, jet needle clip position, pilot jet
and pilot air screw.
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