Yamaha YZ450FY 2009 Owner's Service Manual page 265

Changing from NFLR-3 to NFPR-3 has the
same effect as a rising of 0.5 clip position.
Effects of changing the jet needle (refer-
ence)
(Diameter of the straight portion)
Changing the diameter of the straight portion
adjusts the air-fuel mixture when the throttle is
1/8 to 1/4 open.
A. Idle
B. Fully open
RELATIONSHIP WITH THROTTLE OPENING
The flow of the fuel through the carburetor main
system is controlled by the main jet and then, it
is further regulated by the area between the
main nozzle and the jet needle.
The fuel flow relates to the diameter of the
straight portion of the jet needle with the throttle
1/8 to 1/4 open and relates to the clip position
with the throttle 1/8 to 3/4 open.
Therefore, the fuel flow is balanced at each
stage of throttle opening by the combination of
the jet needle straight portion diameter and clip
position.
ADJUSTING THE LEAK JET (ADJUSTING
THE ACCELERATOR PUMP)
The leak jet "1" is a setting part that adjusts the
flow of fuel discharged by the accelerator
pump. Since the accelerator pump operates
only when throttle is open, the leak jet is used
to adjust a fuel mixture ratio for quick throttle
opening and is therefore different from other
setting parts that adjust a fuel mixture for each
throttle opening (each engine speed).
1. The engine breathes hard in quick throttle
opening, select a leak jet having lower cali-
brating No. than standard to enrich the mix-
ture. <Example> #55 → #50
2. Rough engine operation is felt in quick throt-
tle opening, select a leak jet having higher
calibrating No. than standard to lean out the
mixture. <Example> #55 → #60
Standard leak jet
7-4
ENGINE
#55