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tions
.
The first number
is
the overall
length
of
the
needle, rounded off to the lowest
10
mm
.
Thus a 5
ind icates that
the
needle is at least 50 mm but less
than 60
mm
in length.
The
letters on the needle
indicate the taper of each section. The first
letter
stands for the taper of the top section (closest to
the grooves), and the second letter stands for the
bottom taper.
The
taper angles are graduated in
15'
(15 minute) increments w ith a taper of
15'
indicated
by the
letter
A.
a taper of 30' indicated by
the letter B and so on. A needle
marked
6DH3
would be between 60 and 70 mm with a top taper
of 1
°
and a bottom taper of
2°.
Refer to chart
showing the taper angles of the double taper
needles used by Kawasaki
.
Needles are sometimes
referred to with another number which is not
actually stamped on the needle. This number indi-
cates the groove in which the clip belongs
.
A
6DH3-3 would refer to the needle described, as
well as telling
that
the cl ip should be
in
the third
groove from the
top.
CLEARANCE
Figure 2-22
2 -18
THEORY OF OPERATION
NEEDLE TAPER ANGLES
A
0°
15'
I
20 15'
B
0°
30'
J
2° 30'
e
0° 45'
K
2° 45'
D
10
0'
L
3
0
0'
E
10 15'
M
3° 15'
F
10
30'
N 3° 30'
G
10 45'
0
3° 45'
H
20
0'
P 4°
0'
The top of the needle is
fastened
to the
throttle
slide, and the tapered end exte
nds
into
the
needle
jet. The needle jet fits
in
the carburetor
body in the
center of the venturi. The
lower
end
of
the
needle
jet is fed fuel from the float chamber
by
the
ma in
jet.
The
inside
diameter of
the jet
is
greater than
the
non
-tapered section of
the
needle. The
inside
diameter of the needle
jet used is
2.680
mm and
the outside
diameter
of the
needle
is
2 .512 mm
.
Subtracting the diameter
of the needle
from
the di-
ameter of the
jet gives the amount of clearance
between them
.
2.680
mm
-2.512 mm
0.168 mm
The clearance
is 0.168 mm. As shown in Figure 2-
22,
the
needle
blocks the jet when
the
throttle
slide is
closed
.
When the
slide
is lifted, the needle
rises out of the jet until
the tapered section ar-
rives at the upper
end of
the
jet.
The effect of the
taper
is
to increase the clearance
between the jet
and the needle as the
needle
is
lifted
.
With
the thrott le slide
raised, fuel
from
the float
bowl
is
drawn by
reduced
pressure,
up into
the
jet
and through the clea rance area
between
the
jet
and the needle. The double
taper of
the
needle
serves to accelerate the flow rate
of
the
fuel so
the
amount of
fuel
can
keep
up with
increasing
amount of air
in higher throttle positions.
Chang ing the position of
the
needle
changes the
amount of available fuel. The
needle
is
moved by
moving the clip up or
down in the grooves on the
needle. Moving
the
clip towards
the top of the
needle delays the
fuel
flow increase, leaning
out
the mixture. This
is
known as
lowering the needle
(because
if
the clip goes
up,
the
needle
sits
lower).
Raising the needle (lowering
the
clip) richens
the
mixture by advancing
the
fuel
flow
increase
.
(See
Figure 2 -23.)
Around 3
/
4 throttle, the clearance
between
the
needle and the
jet
becomes greater than
the cross-
sectional area of the main
jet.
At
this
point, the
fuel flow is determined by the
size of
the
main jet.
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