2000 YAMAHA YZ426F fuel

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TUN

SETTING
Pilot jet adjustment
The richness of the air-fuel mixture with the
throttle open 1/4 or less can be set by adjust-
ing the pilot jet.
If the engine breathes hard when you start
opening the throttle, the air-fuel mixture is too
rich and should be made leaner.
Effects of adjusting the pilot jet (reference)

Idle
õ
Fully open
Standard pilot jet #42
1/4 1/2 3/4
+5%
–5%
#45
#40#42

õ

Jet needle groove position adjustment
Adjusting the jet needle
1
position affects the
acceleration when the throttle is 1/4 to 1/2
open.
1. Too rich at intermediate speeds
Rough engine operation is felt and the
engine will not pick up speed smoothly. In
this case, step up the jet needle clip by
one groove and move down the needle to
lean out the mixture.
2. Too lean at intermediate speeds
The engine breathes hard and will not
pick up speed quickly.
Step down the jet needle clip by one
groove and move up the needle to enrich
the mixture.
Effects of changing the jet needle groove
position (reference)

Idle
õ
Fully open
1
No.5 groove
2No.3 groove
3No.4 groove
Standard clip position No.4 groove
1/4 1/2 3/4
+10%
–10% AB
1
23

7 - 6
TUNSETTING
Jet needle adjustment
The jet needle is adjusted by changing it.
* Except for USA
The tapered sections of all jet needles have
the same starting positions, but the needles
are available with different taper angles and
straight-portion diameters.

OBEKR - 4
Clip position
Diameter a of straight portion
Taper angle b Standard jet needleOBEKR
*OBEKQ
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.
IdleõFully open
(Taper)
A smaller taper affects the richness of the air-
fuel mixture when the throttle is fully open. To
change the air-fuel mixture only at 1/2 throttle,
also adjust the main jet.
IdleõFully open
1/4 1/2 3/4
+10%
Q
–10%
R
P
õ
1/4 1/2 3/4
+5%
D
–5%
C
E

õ

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TUN
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 throt-
tle 1/8 to 1/4 open and relates to the taper
starting position and the clip position with the
throttle 1/4 to fully open.
Therefore, the fuel flow is balanced at each
stage of throttle opening by the combination of
the jet needle straight portion diameter, taper
starting position and clip position.
SETTING
Lean (larger diameter)
õRich (smaller diameter)
11/8 throttle
21/4 throttle
31/2 throttle
41/1 throttle
aMain nozzle

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EC71D021
Examples of carburetor setting depending on symptom
This should be taken simply for an example. It is necessary to set the carburetor while checking
the operating conditions of the engine.
Symptom Setting Checking
At full throttle
Hard breathing
Shearing noise
Whitish spark plug
Lean mixtureIncrease main jet calibration no.
(Gradually)Discoloration of spark plug ®
If tan color, it is in good condition.
If cannot be corrected:
Clogged float valve seat
Clogged fuel hose
Clogged fuel cock
Accelerator pump
At full throttle
Speed pick-up stops
Slow speed pick-up
Slow response
Sooty spark plug
Rich mixtureDecrease main jet calibration no.
(Gradually)Discoloration of spark plug ®
If tan color, it is in good condition.
If cannot be corrected:
Clogged air cleaner
Fuel overflow from carburetor
Lean mixture Lower jet needle clip position.
(1 groove down)
The clip position is the jet needle
groove on which the clip is installed.
The positions are numbered from the
top.
Accelerator pump (except for rich mix-
ture symptom). Rich mixture Raise jet needle clip position.
(1 groove up)
1/4 ~ 3/4 throttle
Hard breathing
Lack of speedLower jet needle clip position.
(1 groove down)
1/4 ~ 1/2 throttle
Slow speed pick-up
Poor accelerationRaise jet needle clip position.
(1 groove up)
Closed to 1/4 throttle
Hard breathing
Speed downUse jet needle with a smaller diameter. Slow-speed-circuit passage
Clogged ® Clean.
Overflow from carburetor
Accelerator pump
Closed to 1/4 throttle
Poor accelerationUse jet needle with a larger diameter.
Raise jet needle clip position. (1 groove
up)
Poor response in the low to
intermediate speedsRaise jet needle clip position.
If this has no effect, lower the jet needle
clip position.
Poor response when throttle
is opened quicklyCheck overall settings.
Use main jet with a lower calibration no.
Raise jet needle clip position.
(1 groove up)
If these have no effect, use a main jet
with a higher calibration no. and lower
the jet needle clip position.
Check that the accelerator pump oper-
ates smoothly.Check air cleaner for fouling.
Jet needleClip
Groobe 7 Groobe 6
Groobe 5
Groobe 4
Groobe 3
Groobe 2
Groobe 1
Leaner
­
(Standard)
¯
Richer
*
SETTING