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reservoir and needle are
located
.
At
high
RPM, this
extra
vacuum effect
helps
pull more fuel
up from
the
float
bowl. The
lip
on the needle
jet is known
as a primary choke
.
The height of
the
primary
choke
determines the amount of
turbulence,
and
the
amount of
extra
depression
at the
venturi exit
of the needle
jet.
A taller primary
choke
will
create
more
turbulence
and
suction
,
leading
to
a
richening of
t
he
mixture at higher RPM
.
Using a
taller primary choke allows use
of
a smaller main
jet to achieve approximately the same mixture ratio
at
full throttle but a decrease in fuel consumption
in the medium throttle position.
The needle
jet is ident
ifi
ed
by
its
diameter
.
Figure
2-26 is a chart
for identifying the
size
of needle
jets
of the primary type.
The
last
componen
t
of the main system
is
the main
jet
and
the
metering
screw (not on all car-
buretors).
The main jet
is
attached
to the ins ide of
the float bowl cover
.
An O-ring is
used
to seal the
bottom of the needle jet assembly
to
the
float
cover
to insure that fuel can
enter
the needle
jet
only
through the main
jet
.
The main jet meters all fuel
to the main system
.
When the main system begins
to meter fuel
,
the
throttle slide
cutaway,
needle,
needle
jet, and main
jet determine the amount
of
fuel
that
should
be de
-
livered
for the
amount of air entering
the
engine.
This working relationship continues until approxi
-
mately
3
/
4
throttle
.
At
this point, the cross
-
sectional area of
the
clearance
between the needle
and the needle jet
becomes
greater than
the
cross-
sectional area of
the
main
jet
.
When this
occurs,
there are no
restrictions
to fuel flow any greater
than the restrictions of the main
jet;
and
the
size
of the main jet determines
how
much
fuel will
be
available
.
Main
jet
size
is
indicated by the number stamped
on
the jet. There are different types of main jets,
each
with a different numbering system
.
Figure 2-
27
shows each type of jet and Figure 2-28 com-
pares
their flow rates in cc per minute to the num-
bers stamped on the jets.
Use extreme care
when
changing main jets.
Use
a
replacement of the same type
.
Matching
only the
numbers can
result
in total confusion as to
why
the
engine
will not
run properly.
AMAL
REVERSE
.138
in
.
(3.5
mm)
.256
in.
(6 .5 mm)
Figure
2-27
400r---~--~~--~--'----'--~~--'
300~--~--~~--~---4~--+----+--~
c
~200~--~~--~~4----+----+----+--~
u
u
#100 #150 #200 #250 #300 #350
Figure 2-28
NEED
LE
JET
INSID
E
DIAMETERS (MM)
0
1
2
3
4
5
6
7
8
9
N
2.550
2.555
2.560
2.565
2 .570
2.575
2,580
2.585
2 .590
2.595
0
2.600
2.605
2 .610
2.615
2 .620
2 .625
2 .630
2 .635
2.640
2 .645
P
2.650
2.655
2 .660
2.665
2 .670
2.675
2 .680
2 .685
2.690
2
.695
Q
2
.700
2
.705
2 .710
2 .715
2.720
2.725
2.730
2 .735
2.740
2
.745
Figure 2 -26
2-20
THEORY OF OPERATION
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