Yamaha YZ250(W)/W1, Owner'S Service Manual

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7-6
SETTING
TUN
EC719081
Jet needle adjustment
On the carburetors used in the YZ250, the main
nozzle is press-fitted, so it can not be replaced.
Therefore, carburetor setting requires the change
of the jet needle.
1. The jet needle setting parts, having the
same taper angle, are available in different
straight portion diameters and in different
taper starting positions.
<Example>
N3EW - 3
Clip position
Diameter
a
of straight portion
Taper starting position
b
A
Difference in straight portion dia.
B
Difference in taper starting position
c
Rich
d
Lean
2. Effects of changing the jet needle (reference)
9 Diameter of straight portion
Changing the diameter of the straight por-
tion adjusts the air-fuel mixture when the
throttle is 1/8 to 1/4 open.
9 Taper starting position
<Difference of 0.5 groove>
Rich N3EW-4th groove
N3CW-4th groove
N3EW-3rd groove
N3CW-3rd groove
Lean N3EW-2nd groove
Changing the taper starting position pro-
duces the same effect as changing the clip
position by 0.5 groove.
<Example>
In case of being 0.5 groove leaner in rela-
tion to N3EW-3rd groove, choose N3CW-
3rd groove.
A
In case of being 0.5 groove leaner in relation
to N3EW-3rd groove.
a
Difference of 0.5 groove
Standard jet needle N3EW
N3CW
N3EW
å
a
1
1
2
2 3
3
4 5
4 5
0.5
0.5
0.5
0.5
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