The purpose of the choke system is to provide a richer mixture for cold engine starting and operation. Mixture enrichment is necessary because fuel vapor has a tendency to condense on cold engien parts, such as the inside area of the intake manifold and cylinder head, thereby decreasing the amount of combustible mixture available in the engine cylinder.
The model BC choke system includes a thermostatic coil, choke housing, choke piston, choke valve, fast idle cam and linkage. It is controlled by a combination of manifold vacuum, air velocity against the offset choke valve, and tension of the thermostatic coil.
When the engine is cold, tension of the thermostatic coil holds the choke valve closed. Starting the engine causes air velocity to strike the offset choke valve which tends to open it along with the action of intake manifold vacuum on the choke piston in the choke housing. After the choke vavle is opened to a predetermined position (vacuum break); the tension of the thermostatic coil balances against the forst of air on the choke valve and the pull of the choke piston.
As the engine warms up, the manifold vacuum which exists in the choke housing pulls hot air from the choke stove through an air passage in the choke housing to heat the thermostatic coil.
A baffle plate inside the choke housing, next to the thermostatic coil serves to distribute heat evenly around the coil to prevent hot spots which would cause uneven opening of the choke valve. Through application of exhaust manifold heat, the thermostatic coil relaxes gradually until the choke valve is fully open.
As the engine is accelerated during warm-up, the corresponding drop in manifold vacuum on the choke piston and reduced air flow against the off-set choke valve, allows the thermostatic coil to momentarily close the choke, providing a richer mixture.
During warm-up it is necessary to provide a faster idle to prevent engine stalling. This is accomplished by a fast idle cam which is connected by a rod to a lever on the choke shaft. The idle screw on the throttle lever contacts graduated steps on the fast idle cam to provide a faster idle than normal, to prevent engine stalling during the warm-up period. When the engine is fully warm and the choke valve is wide open, the fast idle cam rotates so the idle screw rests on the low step on the fast idle cam where normal curb idle is obtained.
If the engine becomes flooded during the starting period, the choke valve can be partially opened manually to allow increased air flow through the carburetor. This is accomplished by depressing the accelerator pedal to the floor. The unloader projection on the throttle lever contacts the edge of the fast idle cam and, in turn, partially opens the choke valve. The extra air leans out the fuel mixture enough so the engine will start.
The model BC choke system consists of a choke valve located in the air horn, a vacuum break diaphragm unit, fast idle cam, choke linkage and a thermostatic coil which is located on the engine exhaust manifold. The thermostatic coil is connected to the choke valve by a rod. The choke operation is controlled by a combination of intake manifold vacuum, the off-set choke valve, atmospheric temperature, and exhaust manifold heat.
The thermostatic coil located on the engien manifold is calibrated to hold the choke valve closed when the engine is cold. When starting the engine, air velocity against the off-set choke valve causes the valve to open slightly, against the torque of the thermostatic coil. When the engine is started, manifold vacuum applied to the vacuum diaphragm unit mounted on the carburetor air horn, will open the choke valve to a point where the engine will operate without loading or stalling. The choke valve will remai in position until the engine begins to warm up and the heat from the exhaust manifold warms the thermostatic coil to relax its tension and allows the choke valve to gradually open. Opening of the choke valve is controlled by air flow through the carburetor air horn, past the off-set choke valve and manifold heat acting upon the thermostatic coil.
