Motorcraft Carburetor Chokes

The electric choke system, shown, is an aid in reducing vehicle exhaust emissions by matching choke operation to engine requirements throughout a wider range of conditions. The electric choke consists of an ambient temperature sensing control switch connected in series with a positive temperature coef­ficient (PTC) ceramic heater. For vehicle operation above 60° F., electric current is supplied to the PTC heater. This warms the bimetal choke control thermo­ stat providing a faster choke come-off. For vehicle operation below 60° F., the electric choke system is inactive, and the choke provides enrichment until the engine warms to operating temperature.

Motorcraft Electric Choke

The automatic choke assembly is mounted on the carburetor body. It has a bimetal thermostatic coil which winds up when cold, and unwinds when hot. A vacuum diaphragm and spring controls the initial operation of the choke. Engine coolant, flowing through a choke water cover, heats the bimetal coil and controls the final choke opening. To start the engine, the accelerator pedal is depressed, closing the choke valves. This permits fuel to flow through the main metering system as well as the idle system. When the engine starts, air flows past the off-set choke valves, and manifold vacuum, acting on the choke vacuum diaphragm, opens the choke valves to a predetermined position, As the engine coolant warms up, it circulates through the choke housing, heating the bimetallic choke coil. The coil unwinds, permit­ting full opening of the choke valves. If the cold en­gine is suddenly accelerated, the resulting drop in manifold vacuum on the vacuum diaphragm allows
the choke valves to momentarily close.

The fast idle cam, actuated by the choke rod, con­trols idle speed during engine warm-up. When the choke valves are fully opened. the fast idle cam ro­tates free of the fast idle screw. An unloader tang on the throttle lever partially opens the choke valves when the accelerator is fully depressed. This permits unloading or breathing of a flooded engine. During deceleration there is a high vacuum condition, causing fuel from the bowl to pass through a restriction
where it is mixed with air from another calibrated restriction. This mixture passes through a horizontal passage where it is mixed with a larger quantity of air, and then flows through a vacuum-operated deceler­ation valve into the intake manifold.
Automatic choke

In order to start and operate a cold engine, a richer air-fuel mixture must be introduced into the cold in­take manifold. The need for a richer air-fuel mixture is due to condensation of the atomized fuel inside the cold manifold and resulting poor mixture distribution to the cylinder. The choke system permits a richer air-fuel mixture which is required for starting and operating a cold engine. The choke late is operated either manually or by some automatic means. It may
be closed during the cranking period and partially opened during the warm-up, confining manifold vac­uum below the choke plate. The greater vacuum caus­es both main metering system and idle system to dis­charge fuel into the cylinders.

The manual choke system shown is cable-operated, and the choke plate positions are con­trolled by the driver. The cable, which is pulled out for starting, closes the choke plate. When the engine starts, the driver pushes the cable in halfway. This places the choke plate in the mid-position. When the engine warms up sufficiently, the driver pushes the cable in completely, thus placing the choke plate in the wide-open position.

Manual Choke
When the engine is cold, the torque, exerted by the bimetallic spring, forces the choke linkage to close the choke plate. As the engine starts, a pressure dif­ference, created by intake manifold and atmospheric pressures acting against the offset choke plate, causes
the choke plate to open slightly.

Manifold vacuum,  applied to the choke piston or diaphragm, pulls the piston or dia­phragm downward and further assists in opening the choke plate. The choke plate opens to a position where the tension from the thermostatic spring is bal­anced by the pressure differential force acting on the choke plate and the pull of vacuum on the piston or diaphragm. Enough air is drawn around the choke
plate to prevent engine flooding and to enable the engine to operate.

Manifold vacuum
The piston initial pulldown travel is limited by vac­uum bypass slots in the piston cylinder wall and by a slot in the piston. When the slot in the piston is ex­posed to the slots in the cylinder wall, vacuum travels through the channel, bypassing the piston, reducing the pulldown force, and restricting further downward movement of the piston. The choke plate then is opened a predetermined distance, but no further.

When the engine reaches its normal operating temper­ature, the bimetallic spring exerts torque to hold the choke plate open. If the engine is accelerated during the engine warm-up period, the drop in manifold vac­uum, applied to the choke piston or diaphragm, al­
lows the thermostatic spring to partially close the choke plate for a brief moment. This action provides a richer air-fuel mixture to prevent engine stumble or stalls.

If the engine should flood (over-rich mixture) during the starting period, the choke plate may be opened manually to lean out the excessively rich air-fuel mix­ture in the intake manifold. This is accomplished by fully depressing the accelerator pedal and engaging the starter. A projection on the throttle lever contacts the unloader face on the fast idle cam and, in turn, partially opens the choke plate.

During the engine warm-up period, it is necessary to provide a faster idle speed to prevent engine stalling. A fast idle cam is rotated into position by the automatic choke lever. A fast idle adjusting screw on the throttle lever contacts the cam face to provide the increased engine speed that is required.

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