FUEL
SYSTEM
boss, forming the upper part of the suction
chamber casting
An
extremely accurate fit is provided
between the spindle and the bush in the
suction chamber so that the enlarged
portion of the piston is held out of
contact with the bore of the suction
chamber, within which, nevertheless, it
operates with an extremely fine clearance.
Similarly, the needle (6) is restrained from
contacting the bore of the jet
(5)
which it
is seen to penetrate, moving axially therein
to correspond with the rise and fall of
the piston.
It will be appreciated that, as the piston
rises, the
air
passage in the neighbourhood
of the jet becomes enlarged, and passes an
additional quantity of
air.
Provided that
the needle (6) is of a suitably tapered form,
its simultaneous withdrawal from the jet
(5)
ensures the delivery to the engine of the
required quantity of fuel corresponding to
any given position of the piston
and hence
to a given
air
flow.
The piston, under the influence of its own
weight and assisted by the light compression
spring (8)
will
tend to occupy its lowest
P
osition, two slight protuberences on its
ower face contacting the bottom surface of
the
main
air
passage adjacent to the jet.
The surface in this region is raised some-
what above the general level of the main
bore of the carburettor, and is referred to
as the
"
bridge" (28).
Levitation of the piston is achieved by means
of the induction depression, which takes
effect within the suction chamber, and thus
upon the upper surface of the enlarged
portion of the piston through
drillings in
the lower part of the piston which make
communication between this region and
that lying between the piston and the
throttle.
The annular space beneath the
enlarged portion of the piston is completely
vented to atmosphere by ducts not indicated
in the
diagram.
It
will
be appreciated that, since the weight
of the piston assembly is constant, and the
augmenting load of the spring
(8)
approxi-
mately so,
a
substantially constant degree
of depression
will
prevail within the suction
chamber, and consequently in the region
between the piston and the throttle, for
any given degree of
lift
of the piston between
the extremities of its travel.
16
It
will
be clear that this floating condition
of the piston
will
be stable for any given
air-
flow demand as imposed by the degree of
throttle opening, the engine speed and the
load
;
thus, any tendency in the piston to
fall momentarily
will
be
accompanied by
an increased restriction to
air
flow in the
space bounded by the lower side of the
piston and the bridge,
and this
will
be
accompanied by a corresponding increase in
the depression between the piston and
throttle, which is immediately communicated
to the interior of the suction chamber,
instantly counteracting the initial disturb-
ance by raising the piston to an appropriate
extent.
The float chamber, which is shown in
Fig.
SA, is of orthodox construction, com-
prising a needle valve
(9)
located within a
separate seating which, in turn, is screwed
in the float chamber lid, and a float (10),
the upward movement of which, in response
to the rising fuel level, causes final closure
of the needle upon its seating through the
medium of the hinged fork (1
l).
The float-chamber is a unit separate from
the main body of the carburettor to which
it is attached by means of the bolt (12),
suitable drillings being provided therein
to lead the fuel from the lower part
of the float chamber to the region surround-
ing the jet. It is steadied at its upper
extremity by a suction chamber attachment
screw.
The buoyancy of the float, in conjunction
with the form of the lever
(11)
is such
that a fuel level is maintained approximatelj
3"
below the jet bridge
(see page 23)
This
can
easily be observed after firs1
detaching the suction chamber and suctior
piston, and then lowering the jet to it:
full
rich position.
The level
can
vary
:
further
f"
downwards without any
il
effects on the functioning of the carburettor
The only parts of importance in Figs.
I
and 8A not so far described are thosl
associated
with
the jet.
Under idling conditions the piston
i
completely dropped, being then supportet
by the two small protuberances provided o
its lower surface, which are in contact
wit
the bridge (28)
;
the small gap thus forme
between piston and bridge permits
th
flow of sufficient
air
to meet the
idh
demand of the engine without, howeve
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