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10-24
GROUP
10-FUEL SYSTEM
OPERATION
FIG. 1 -
3/4 Left Rear View
Fuel enters the carburettor at the fuel inlet,
flowing through the float needle valve and seat into
the float chamber. When the fuel reaches a
predetermined level, the float shuts off the fuel
supply
by closing the needle valve against its seat.
The needle valve is equipped with a viton
rubber tip to provide increased service life
and
greater tolerance of any dirt particles that may
enter
with the fuel.
As previously mentioned, the float chamber is
vented internally through a passage and vent tube
whenever the engine is running above idle speed.
(Internal venting ensures that any reduction of air
pressure in the air horn, due to a restricted air
cleaner, is balanced by a similar reduction in the air
pressure
acting
on the fuel in the float chamber.
The
fuel/air
mixture ratio will therefore remain
unchanged under these
conditions).
When the engine is stopped or whilst idling,
the float chamber is vented to atmosphere via the
opening in the air horn, through which the
accelerating pump piston stem
passes.
This opening is normally closed by a metal
vent washer which is lifted clear of the vent orifice
by
a small
pin in the pump piston stem as the
throttles return to slow-idle position. The purpose
of the external vent is to permit the escape uf fuel
vapours formed in the float chamber under high
temperature operating conditions. Additionally,
the throttle body has two small vents, one in each
barrel to permit the escape of fuel vapours which
may
accumulate
in this area. (Refer Fig. 2.)
FLOAT
SYSTEM (Fig. 2)
IOU DISCHARGE HOU5
IDLE SYSTEM (Figs. 2 and 3)
Identical idle systems supply each barrel with
the
correctly
proportioned
fuel/air
mixture
required for idling operation.
The throttle valves are held open to the correct
slow-idle position by the slow idle adjustment
screw. By turning this screw the throttle valve
opening is varied
.
thereby controlling the amount
of
air entering the engine and regulating the idle
speed
.
(A single barrel only will be considered in
the following explanation).
tOil DISCHARGE HOlES
FIG. 3
With the throttle valve closed to idle position,
engine
depression
is
concentrated
on
the
idle
discharge nozzle located below the throttle
valve.
Consequently, fuel flows from the base of the main
discharge jet through the idle tube (Slow-Running Jet)
where it is metered by a calibrated orifice in the base
of the tube.
The metered fuel mixes with air taken in through
the idle air bleed, and the resulting fuel/air emulsion
passes down the idle channel where it mixes with
additional air passing through the
secondary
idle air
bleed and through the two upper idle discharge holes.
The mixture finally passes from the idle discharge
nozzle into the air passing the partly open throttle.
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