FUEL
SYSTEM
creating enough depression on the induction
side to raise the piston.
The fuel discharge required from the jet
is very small under these conditions,
hence the diameter of the portion of the
needle now obstructing the mouth of the
jet is very' nearly equal to the jet bore.
Initial manufacture of the complete carb-
urettor assembly to the required degree of
accuracy to ensure perfect concentricity
between the needle and the jet bore under
these conditions is impracticable, and an
individual adjustment for this essential
centralisation is therefore provided.
It
will be seen that the jet is not mounted
directly in the main body, but is housed in
the parts (13) and (14) referred to as the
jet bushes, or jet bearings.
The upper jet bush is provided with a
flange which forms a face seal against a
recess in the body, while the lower one
carries a similar flange contacting the upper
surface of the hollow hexagon locking nut
(15)-
The arrangement is such that tightening of
the hollow hexagon locking screw will
positively lock the jet and jet bushes in
position. Some degree of lateral clearance
is provided between the jet bushes and the
bores formed in the main body and the
locking screw. In this manner the assembly
can
be moved laterally until perfect con-
centricity of the jet and needle is achieved,
the screw (15) being slackened for this
purpose. This operation is referred to as
"
centring the jet
",
on completion the jet
locking nut (15) is finally tightened. See
page 19.
In addition to this concentricity adjustment,
an
axial
adjustment of the jet is provided
for the purpose of regulating the idling
mixture strength.
Since the needle tapers throughout its
length, it
will be clear that raising or lower-
ing the jet
within
its bearing
will
alter the
effective aperture of the jet orifice, and
hence the rate of fuel discharge. To permit
this adjustment the jet is a variably mounted
within its bearings and provided with
adequate sealing glands.
A
compression spring (16) which, at its
upper end, serves to compress the small
sealing gland
(17)
and thus prevents any
fuel leakage between the jet and the upper
jet bearing.
At its lower end this spring abuts against a
similar sealing gland, thus preventing leak-
age of fuel between the jet and the lower
jet bearing.
In both locations a brass washer is inter-
posed between the end of the spring and the
sealing gland to take the spring thrust.
A
further sealing gland (19), together with
a conical brass washer (20) is provided, to
prevent fuel leakage between the jet screw
(15) and the main body.
It
will
be seen from the diagram that the
upward movement of the jet is determined
by the position of the jet adjusting nut (18)
since the enlarged jet head (21) finally
abuts against this nut as the jet is moved
upwards towards the
"
weak
"
or running
position.
The position of the nut (18) therefore
determines the idling mixture ratio setting
of the carburettor for normal running with
the engine hot, and is prevented from
unintentional rotation by means of the
loading spring (22).
The cold running mixture control mechan-
ism comprises the jet lever (23) supported
from the main body by the
link
member
(24) and attached by means of a clevis pin
to the jet head (21).
A
tension spring (25)
is provided, as shown, to assist
in
returning
the jet-moving mechanism to its normal
running position. Connection is made
from the outer extremity of the jet lever
(23) to a control situated
within
reach of
the driver.
Drillings in the float-chamber attachment
bolt (12), the main body of the carburettor,
the jet (5) and slots in the upper jet bearing
(13) serve to
conduct the fuel from the
float-chamber to the jet orifice.
It
will
be seen that the spindle upon which
the piston (3) is mounted is hollow, and
that -it surr&nds a small stationary damper
is ton
sus~ended from the suction chamber
bp
by m& of the rod (26). The hollow
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