Choke Systems; Integral Automatic Choke - Ford LSG 875 Service Manual

3-25
FUEL SYSTEM 3-25
MODEL 4160 CARBURETOR
SYSTEMS (Continued)
CHOKE SYSTEMS
The choke system provides the richer mixture
required to start and run a cold engine. A fully or
partially closed choke valve creates a vacuum
below the valve so that fuel is discharged from
both the idle and main metering systems. Clos-
ing the choke plate results in greater enrich-
ment. The richer mixtures are needed because
the cold fuel will not vaporize readily and much
of the fuel condenses on the walls of the cold
intake manifold.
The integral choke has the bimetal assembly
and vacuum piston housing mounted on the car-
buretor. In this model the heat required to oper-
ate the bimetal is supplied from an intake
manifold cross-over tube or a heat stove
mounted on or in the exhaust manifold.
Some of these systems may use an electric
heating element to assist and speed. up choke
" come off" time to meet tighter emission
requirements.
A fast idle cam and linkage permits higher en-
gine RPM when the engine is cold. An unloader
tab is provided on the throttle lever will mechan-
ically open the choke plate when the throttle lev-
er is held at the wide open position. This permits
the operator to relieve an over-choked or
flooded condition. Some applications may delete
the fast idle cam feature.
Electric chokes, where the bimetal assembly
is heated only by an electrical resistor are be-
coming more popular for certain applications.
Hand operated chokes are used on many spe-
cial applications.
INTEGRAL AUTOMATIC CHOKE
When the accelerator pedal is depressed and
released the choke thermostatic spring pushes
the choke valve to a fully or partially closed posi-
tion and the fast idle cam to a corresponding fast
idle position. A cold engine at lower tem-
peratures will allow the choke valve to close
completely and the fast idle cam to move to its
top step.
- -
- -
A manifold vacuum passage through the car-
buretor body is connected to the bottom end of
the choke piston cavity. When the engine starts,
the vacuum acting on the bottom of the choke
piston, opens the valve to a predetermined posi-
tion established by the " choke piston travel ad-
justable stop." This initial vacuum opening is
called the " choke qualifying dimension," " vac-
uum break,"
" vacuum kick" or " vacuum pull
down ."
The choke plate may open further under in-
creasing airflow conditions.
The choke heat tube, usually asbestos cov-
ered, connects the heat source with the heat
tube connection of the choke housing. After the
piston is pulled down to its stop, a vacuum by-
pass slot or hole in the center of the piston is
opened up and increased hot air is circulated in
the housing to further heat the bimetal spring. As
the spring warms up, its tension on the choke
shaft will be relaxed and allow the choke valve
to open gradually to the wide open position. The
choke bimetal cover has index marks and an ar-
row indicating the lean direction. Different ap-
plications require different settings. When the
engine first starts, the fast idle cam can be
" kicked down" to a lower step and speed by tap-
ping the throttle lever lightly.
1,
PISTON AND
HOUSING
PLATE
VACUlh PASSAGE
I
THERMOSTAT
SPRING
%