Yamaha 2011 YZ250FA Owner's Service Manual page 266

TUNING
ENGINE
CARBURETOR SETTING
• The air/fuel mixture will vary depending on at-
mospheric conditions. Therefore, it is neces-
sary to take into consideration the air
pressure, ambient temperature, humidity,
etc., when adjusting the carburetor.
• Perform a test run to check for proper engine
performance (e.g., throttle response) and
spark plug(-s) discoloration or fouling. Use
these readings to determine the best possible
carburetor setting.
It is recommended to keep a record of all car-
buretor settings and external conditions (e.g.,
atmospheric conditions, track/surface condi-
tions, lap times) to make future carburetor set-
ting easier.
• The carburetor is a part of the fuel line.
Therefore, be sure to install it in a wellven-
tilated area, away from flammable objects
and any sources of fire.
• Never look into the carburetor intake.
Flames may shoot out from the pipe if the
engine backfires while it is being started.
Gasoline may be discharged from the ac-
celerator pump nozzle when the carbure-
tor has been removed.
• The carburetor is extremely sensitive to
foreign matter (dirt, sand, water, etc.). Dur-
ing installation, do not allow foreign mat-
ter to get into the carburetor.
• Always handle the carburetor and its com-
ponents carefully. Even slight scratches,
bends or damage to carburetor parts may
prevent the carburetor from functioning
correctly. Carefully perform all servicing
with the appropriate tools and without ap-
plying excessive force.
• When the engine is stopped or when riding
at no load, do not open and close the throt-
tle unnecessarily. Otherwise, too much
fuel may be discharged, starting may be-
come difficult or the engine may not run
well.
• After installing the carburetor, check that
the throttle operates correctly and opens
and closes smoothly.
ATMOSPHERIC CONDITIONS AND
CARBURETOR SETTINGS
Air Humidi-
temp. ty
High High
Low Low
The air density (i.e., concentration of oxygen in
the air) determines the richness or leanness of
the air/fuel mixture.
• 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.
EFFECT OF SETTING PARTS IN RELATION
TO THROTTLE VALVE OPENING
A. Closed
B. Fully open
1. Pilot screw/pilot jet
2. Throttle valve cutaway
3. Jet needle
4. Main jet
7-1
ENGINE
Air
pres-
sure Mixture Setting
(alti-
tude)
Low
Richer Leaner
(high)
High
Leaner Richer
(low)