Yamaha YZ450FY 2009 Owner's Service Manual page 263

CONSTRUCTION OF CARBURETOR AND
SETTING PARTS
The FLATCR carburetor has a primary main jet.
This type of main jet is perfect for racing ma-
chines 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 manufactured with a
pilot screw. The pilot screw adjustment ranges
from fully closed throttle to 1/4 open throttle.
1. Jet needle
2. Pilot air jet
3. Needle jet
4. Main jet
5. Pilot jet
6. Pilot screw
ADJUSTING THE MAIN JET
The richness of the air-fuel mixture at full throt-
tle can be set by changing the main jet "1".
Standard main jet
If the air-fuel mixture is too rich or too lean, the
engine power will drop, resulting in poor accel-
eration.
Effects of changing the main jet (reference)
A. Idle
B. Fully open
1. #162
2. #158
3. #160
ADJUSTING THE PILOT SCREW
The richness of the air-fuel mixture with the
throttle fully closed to 1/4 open can be set by
turning the pilot screw "1". Turning in the pilot
screw will make the mixture lean at low speeds,
and turning it out will enrich it.
Standard pilot screw
position (example)
• If the engine idling speed fluctuates, turn the
pilot screw only 1/2 of a turn in either direction.
• To optimize the fuel flow at a smaller throttle
opening, each machine's pilot screw has
been individually set at the factory. Before ad-
justing the pilot screw, turn it in fully and count
the number of turns. Record this number as
the factory-set number of turns out.
#160
7-2
ENGINE
1-1/2
7