Yamaha YZ125X 2022, Owner'S Service Manual

Page: 252 / 268

ENGINE
10-1
EAM20198
ENGINE
EAM30590
CARBURETOR SETTING
•
The role of fuel is to cool the engine, and in the
case of a 2-stroke engine, to lubricate the en-
gine in addition to power generation. Accord-
ingly, 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 condi-
tions 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 vehicle for conditions (pick-up of
engine speed, road surface conditions) and for
the discoloration of the spark plug(s).
After taking these into consideration, he must
select the best possible carburetor settings.
TIP
It is advisable to make a note of settings, atmo-
spheric conditions, road surface condition, lap-
time, etc. so that the memorandum can be used
as a reference useful for future.
EAM30591
ATMOSPHERIC CONDITIONS AND
CARBURETOR SETTINGS
The air density (i.e., concentration of oxygen in
the air) determines the richness or leanness of
the air/fuel mixture. Therefore, refer to the above
table for mixture settings.
That is:
• Higher temperature expands the air with its re-
sultant reduced density.
• Higher humidity reduces the amount of oxygen
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.
EAM30592
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 discolor-
ation of spark plug(s).
Air
temp. Humidity
Air
pressure
(altitude)
Mixture Setting
High High
Low
(high)
Richer Leaner
Low Low
High
(low)
Leaner Richer
Discoloration Condition of spark plug
Normal
Insulator is dry and burnt
brown.
Over burned
(too lean)
Insulator is whitish.
Oil fouled
(too rich)
Insulator is sooty and wet.
A. Normal
B. Over burned (too lean)
C. Oil fouled (too rich)
A
B
C