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mounting gives a faster and more
accurate reaction
of the choke to changes in manifold
temperature.
The bi-metallic thermostat spring
in the choke
control mechanism will expand when
cold and con
tract when warm. When the engine
is cold, the
thermostat spring expands, holding
the choke
plates in the closed position. When
the engine is
started, manifold vacuum acts directly
on the
choke plates, immediately moving
them against
the tension of the thermostat spring
to a partially
open position to prevent stalling.
The choke
shaft
does not pass directly through the
center of the
choke plates. Instead, it is slightly
offset, thus
exposing a larger area at one side
of the closed
choke plates to manifold vacuum.
It is this offset
feature which enables manifold vacuum
to move
the choke plates toward the open
position. Mani
fold 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 plates.
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 filtered air from the
carburetor
through the heat tube in the intake
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 con
tract. This gradually decreases
the tension of
the thermostat spring as manifold
temperature
rises, permitting the vacuum acting
on the choke
vacuum piston to further open the
choke plate.
The air then flows through the manifold
vacuum
If the engine should approach a
stall at any
time during the warm-up period,
manifold vacuum
will become weak. The tension of
the thermostat
spring then overcomes the reduced
force acting on
the vacuum piston and the choke
plates will be
moved toward the closed position,
providing a
richer mixture to allow the engine
to "catch" and
run smoothly again.
When the engine reaches its normal
operating
temperature, the thermostat spring
no longer
exerts an opposing tension on the
choke vacuum
piston, allowing the vacuum piston
to pull the
choke
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 plates
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
plates acts upon
the plates 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 plates closer
to the open
position. The offset choke plates,
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 divorced choke is easily accessible
and
provides no additional service
problems. The
choke assembly should be overhauled
at the same
time the carburetor is overhauled
to assure the
improved operation of this type
choke.
7. THE DASHPOT
On Mercury vehicles equipped with
automatic
transmissions, a dashpot is incorporated
with the
carburetor to prevent loading of
the engine when
the accelerator pedal is suddenly
depressed and
released. The dashpot accomplishes
this by re
tarding the closing rate of the
throttle plates in
the last few degrees of their rotation
approaching
the closed or idle position. Slowing
the rate of
travel of the throttle plate gives
the engine time
to dissipate the charge of raw
accelerating fuel
in the intake manifold. A check
valve added to
the dashpot system allows quicker
recovery by
the dashpot, thus giving anti-stall
protection even
for sudden repeated deceleration.
This improved
dashpot system will also open the
throttle plates
during idle should manifold vacuum
approach the
stalling point of the engine. The
dashpot will then
hold the throttle plates open until
sufficient mani
fold vacuum is again developed
to allow the plates
As the accelerator pedal is released,
the
throttle return spring in the accelerator
pedal
linkage acts to close the throttle
plates simul
taneously with the release of the
accelerator pedal.
When the throttle plates approach
the closed or
idle position, the head of the
dashpot adjusting
screw in the dashpot lever contacts
the diaphragm
-10-
