Yamaha YZ250F X 2008 Owner'S Service Manual Download

Page: 181 / 192

7-1

ENGINE

TUNING

ENGINE

CARBURETOR SETTING
• The air/fuel mixture will vary de-

pending on atmospheric conditions.
Therefore, it is necessary to take
into consideration the air pressure,
ambient temperature, humidity,
etc., when adjusting the carburetor.

• Perform a test run to check for prop-

er engine performance (e.g., throt-
tle response) and spark plug(-s)
discoloration or fouling. Use these
readings to determine the best pos-
sible carburetor setting.

It is recommended to keep a record of
all carburetor settings and external
conditions (e.g., atmospheric condi-
tions, track/surface conditions, lap
times) to make future carburetor set-
ting easier.

• The carburetor is a part of the

fuel line. Therefore, be sure to in-
stall it in a wellventilated area,
away from flammable objects
and any sources of fire.

• Never look into the carburetor in-

take. 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
carburetor has been removed.

• The carburetor is extremely sen-

sitive to foreign matter (dirt,
sand, water, etc.). During instal-
lation, do not allow foreign mat-
ter to get into the carburetor.

• Always handle the carburetor

and its components carefully.
Even slight scratches, bends or
damage to carburetor parts may
prevent the carburetor from
functioning correctly. Carefully
perform all servicing with the ap-
propriate tools and without ap-
plying excessive force.

• When the engine is stopped or

when riding at no load, do not
open and close the throttle un-
necessarily. Otherwise, too
much fuel may be discharged,
starting may become 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

The air density (i.e., concentration of
oxygen in the air) determines the rich-
ness or leanness of the air/fuel mix-
ture.
• Higher temperature expands the air

with its resultant 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 altitude) reduces the density of
the air.

EFFECT OF SETTING PARTS IN
RELATION TO THROTTLE VALVE
OPENING

Closed

Fully open

Pilot screw/pilot jet

Throttle valve cutaway

Jet needle

Main jet

CONSTRUCTION OF
CARBURETOR AND SETTING
PARTS
The FLATCR carburetor has a prima-
ry main jet. This type of main jet is
perfect for racing machines since it
supplies an even flow of fuel, even at
full load. Use the main jet and the jet
needle to set the carburetor.
The FLATCR carburetor is manufac-
tured with a pilot screw. The pilot
screw adjustment ranges from fully
closed throttle to 1/4 open throttle.

Jet needle

Pilot air jet

Needle jet

Main jet

Pilot jet

Pilot screw

ADJUSTING THE MAIN JET
The richness of the air-fuel mixture at
full throttle can be set by changing the
main jet "1".

If the air-fuel mixture is too rich or too
lean, the engine power will drop, re-
sulting in poor acceleration.

Effects of changing the main jet
(reference)

Idle

Fully open

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