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 plate 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.
We have 4 different types of chokes, Intregal (or integrated), Divorced, Electric & Electric Conversion Kits.
You can find listings for OE style chokes and conversion kits by make and model in the catalog here.
We do not stock manual choke kits, manual choke parts, or manual to electric conversions. This information is for educational purposes only.
The manual choke system in 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 engien warms up sufficiently, the driver pushes the cable in completely, thus placing the choke plate in the wide-open position.
The integral choke is mounted on the carburetor itself. either off the side of the bowl or throttle body. These are also called “automatic chokes” The illustration below shows the choke mounted on the top of the carburetor. The thermostat is heated by a heat tube that runs down to the exhaust manifold. The heat tubes heats up the thermostat and moves the choke valve to the open position. A vacuum passage in the carburetor feeds up to the choke housing and helps pull in the heat from the heat tube. Some of these chokes will also have a hot water jacket running through, or on the thermostat to facilitate heating.
With this type of choke, the choke is not opened by the thermostat. For the most part when warm, the choke opens from the weight of the linkage hooked to it. Along with that, the vacuum from inside the carburetor pulls it open.
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 off 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.
When the engine reaches its normal operating temperature, the bimetallic spring exerts torgue 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, allows 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. (This is a generalized description, not every carb will be exactly the same).
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.
These images show some various carb types with integrated chokes. Your carburetor will have a bimetal choke housing with either a metal cap/coil or a black plastic cap/coil. These heat style integrated chokes use a tube which transfers heat from the manifold to the choke housing through a port similar to those in the below images. As the carb heats, the coil inside the housing relaxes, allowing the choke flap to spring open, letting more air into the carburetor.
One of the number one issues we run into is damaged, broken, rotted, rusted heat tubes and manifold ports. The tubes are not available aftermarket that we’ve been able to source consistantly, and we typically recommend installing an electric choke conversion kit at this point.
The electric choke system, 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 60F, electric current is supplied to the PTC heater. This warms the bimetal choke control thermostat provided a faster choke release. For vehicle operation below 60F. The electric choke system is inactive, and the choke provides enrichment until the engine warms to operating temperature.
This is a typical electric choke mounted on the carburetor, which is integrated (metal housing on the carburetor body itself). In this case there is one wire on the choke thermostat. This is the 12v source, which all electric chokes use. The thermostat is grounded via the carburetor itself. Some electric chokes will have a 2nd wire (ground wire), which is connected back to the carburetor. When the key is on the thermostat is heated up, opening the choke valve. Many later model carburetors came with electric chokes stock, and we do sell replacements for them. This style would not have an open port on the side of the housing for heat transfer. You can see on the left side of this image the port, which is capped off.
The divorced choke type has the thermostat mounted in the intake manifold instead of on the carburetor. In this illustration, the arrow points to the thermostat which is covered by a metal shield. A rod connects the thermostat to the choke lever, which controls the choke valve to be opened, or closed. As the intake manifold heats up, the thermostat coil expands, opening the choke valve.
The part you can’t see under the shield would look something like this:
