Engine; Carburetor Setting; Atmospheric Conditions And Carburetor Settings; Test Run - Yamaha YZ250X 2022 Owner's Service Manual

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
Air
pres-
Air
Humidity sure
temp.
(alti-
tude)
Low
High High
(high)
High
Low Low
(low)
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
Mixture Setting
Richer Leaner
Leaner Richer
10-1
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).
Discoloration Condition of spark plug
Insulator is dry and burnt
Normal
brown.
Over burned
Insulator is whitish.
(too lean)
Oil fouled
Insulator is sooty and wet.
(too rich)
A
B
C
A. Normal
B. Over burned (too lean)
C. Oil fouled (too rich)
ENGINE