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the flow of fuel. Sufficient fuel to
replace the fuel
that is used will then be the only
fuel admitted.
Thus, any change in the fuel level,
causes a cor
responding movement in the float, opening
or
closing the fuel inlet needle valve
to immediately
restore the proper fuel level. The
specified level
of fuel is maintained because the
four basic fuel
metering systems are calibrated to
deliver the
proper mixture only when the fuel
is at the level.
2. CHOKE
The Model 2110 Truck Carburetors are
equip
ped with a manual choke, the Model
2110-EEC
Passenger Carburetors are equipped
with an auto
matic choke. The choke supplies the
richer fuel
air mixture required for starting
and operating a
cold engine. Most of the vaporized
fuel from the
carburetor condenses to a liquid upon
contact with
the cold surfaces of the intake manifold.
This fuel
in liquid form burns too slowly and
incompletely
in the cylinders, causing loss of
power and stalling.
The choke plate, which is closed during
the warm
up and choking period, confines the
manifold vacu
um below the plate. This greater vacuum
causes
both the main metering and idle system
to dis
charge fuel into cylinders.
Sufficient fuel is
then
supplied to the cylinders.
The automatic choke is mounted on
the air
horn and is linked to the choke shaft.
It controls
the air flow into the carburetor.
The bi-metallic thermostat spring
in the
choke control mechanism will expand
when cold,
loosening and unwinding its coils.
When warm, it
will contract, winding the coils tighter.
When the
engine is cold, the thermostat spring
expands,
holding the choke plate in the closed
position.
When the engine is started, manifold
vacuum acts
directly on the choke plate, and a
vacuum piston
located in the choke housing, immediately
moving
against the tension of the thermostat
spring
to partially open the choke plate
to prevent stalling.
The choke shaft does not pass through
the center
of the choke plate. Instead, it is
offset, thus ex
posing a much larger area at one side
of the closed
choke plate to manifold vacuum. When
the engine
is started or at idle, manifold vacuum
is not suf
ficiently strong to open the choke
plates, but the
impact of air against the choke plate
partially opens
the plate.
Manifold vacuum channeled through
a passage
in the choke control mechanism acts
to draw the
choke vacuum piston downward, thus
exerting
another opening force upon the choke
plate. These
two features allow enough air to
enter the engine
to enable it to run smoothly.
As the engine continues to run, the
vacuum
acting on the choke vacuum piston
draws air from
under the intake manifold through
the heat tube
in the manifold where the air is
warmed by the
engine heat, and then through the
thermostat
housing where the air warms the thermostat
spring, causing it to contract. This
gradually de
creases the tension of the thermostat
spring as
manifold temperature rises, permitting
the vacu
um acting on the choke vacuum piston
to further
open the choke plate. The air then
flows through
the manifold vacuum passage in the
carburetor
and is exhausted into the air stream
in the throttle
body.
When the engine reaches its normal
operating
temperature, the thermostat spring
no longer ex
erts an opposing tension on the choke
vacuum
piston, allowing the vacuum piston
to pull the choke
plate to the full open position.
In the full open position the vacuum
piston is
in its lowest position in the cylinder.
Slots in the
cylinder wall permit sufficient air
to bleed past
the piston and into the intake manifold
to allow a
continual flow of warm air to pass
through the
thermostat housing. This keeps the
thermostat
spring warm and the choke plate fully
open until
the engine is shut down and allowed
to cool.
During the warm-up period, the air
flow past
the partially opened offset choke
plate acts upon
the plate in much the same manner
as manifold
vacuum does upon starting. As air
flow increases
with increased engine speed, the
engine requires
less choking and the force of the
increased air
flow holds the choke plate closer
to the open
position. The offset choke plate,
vacuum piston,
and thermostat spring are engineered
to provide
the correct degree of choking for
all conditions
of engine speed, power output, and
temperature.
The choke lever at the carburetor
actuates a
fast idle cam during choking. Designed
to increase
the idle rpm for smoother running
when the engine
is cold, the fast idle cam has a
series of steps on
one edge. As the choke lever is moved
through
its range of travel from the closed
position to the
fast idle cam rotates, presenting
successive steps
to a throttle stop screw. Each step
permits a
slower idle rpm as engine temperature
rises and
choking is reduced.
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