In a cold engine the fuel condenses out of the fuel / air mixture onto the passageways of the intake manifold and walls of the combustion chamber. Fuel will not burn unless it is mixed with oxygen. To make up for that loss through condensation more fuel needs to be introduced into the engine. To accomplish this a choke, an air valve placed above the main nozzle, shuts off all of the air going into the engine through the carburetor inlet. This allows the full force of the drop in atmospheric pressure (suction) in the engine to operate on the main nozzle and on the idle ports so the engine gets the excess fuel it needs to replace the fuel that has condensed.
One of two types of choke are commonly used on carburetor-equipped engines. A manual choke that is operated by the driver via a pull cable mounted somewhere on the dashboard. The other an automatic choke that uses a variety of mechanisms to accomplish the same thing.
The manual choke requires the driver’s attention to close or open it. Making the automatic choke work though is not usually a problem because the choke butterfly is held closed when the engine is cold by spring tension against the linkage inside the choke housing. The choke butterfly opens as heat from the engine in earlier style chokes relaxed the tension on that spring. An electric choke uses a heating element in the choke housing cover to relax the spring.
The spring alone is not always enough to hold the choke butterfly open when the engine has reached operating temperature. The butterfly does not pivot on center but is offset so that the air stream flowing into the carburetor pushes against the butterfly. The third force holding the choke butterfly open when the engine reaches operating temperature is a piston located inside the choke housing that is pulled down by engine vacuum ported to it from below the carburetor butterfly.
The driver should not pump the throttle before starting the engine and the throttle should never go to wide open on a cold engine. To prevent this a carburetor equipped with an automatic choke has an unloader. This is a little tang on the linkage that opens the choke butterfly enough to clean out the engine when the throttle is pushed to wide open position.
While the choke is operating the engine cannot use the extremely rich mixture unless it is run at a faster than normal idle speed. A fast idle cam and linkage that work in conjunction with the other choke mechanisms raise the idle speed anytime that the choke is working.
Rotating the choke cover, increasing or relaxing the tension on the spring prior to it heating up varies timing of the opening of the choke butterfly. Position of the choke butterfly when fully closed and the position of the piston in the choke housing are adjusted by bending the linkage. Adjusting the choke varies between models of carburetor so the instructions supplied with Edelbrock’s kit or the one packaged with a carburetor rebuild kit should contain the necessary information.
|This early Carter AFB 4-barrel carburetor is equipped with an automatic choke that is actuated by heat transmitted from the exhaust manifold via the tube entering the choke housing. No exhaust gas is used in the process, just heat radiated via the tubing. A piston activated by vacuum, located inside the choke housing, assists in holding the choke butterfly open when the engine has reached operating temperature.|
|Shown are all of the items included in the Edelbrock electric choke kit (#1478) for converting their manual choke equipped Performer series carburetors to electric choke. It was not created for use on the AFB but parts from the kit make the conversion possible.|
|These items from the kit are not used when converting an AFB carburetor from an exhaust / engine-heat activated choke to an electrically actuated choke.|
|The pivot shaft and linkage from the AFB (above) are used instead of the ones (below) supplied in the Edelbrock kit.|
|This view of the back side of the new Edelbrock choke housing with the old AFB pivot shaft arm and linkage installed shows that the arm points up and opens the choke butterfly as it swings to the rear. O-ring (arrow) fits in the recess in the choke housing and seals the vacuum supply passage on the carburetor base.|
|This view of the back side of the new Edelbrock choke housing with the linkage supplied in the kit installed shows that the arm points down and opens the choke butterfly as it swings to the front. This is the way that the choke linkage travels in Edelbrock Performer carburetors.|
|Vacuum is supplied to the piston in the choke housing through this passage (arrow) that is plugged with a lead seal on carburetors without a choke like these AFB carburetors originally intended for racing.|
|Lead seal (arrow) can be removed from the passageway by inserting a knife blade under the edge and lifting the plug up. Brass insert in passage is stock item in stock carburetors.|
|Piston in the choke housing is at the bottom of its travel and the linkage (arrow) at the rearmost point of its travel when the choke butterfly is open.|
|Vacuum from beneath the venturi in the carburetor has drawn the piston down in the choke housing keeping the choke butterfly open once the engine has reached operating temperature.|
|Electric choke uses the same type of coiled spring cover as the one in the older style engine-heat activated choke that relaxes the spring, uncoiling as it is heated. However, instead of using heat from the exhaust manifold or intake, the electric choke uncoils the spring via an electric heating element in the cover. Baffle plate and gasket serve to seal and separate the choke housing from the cover.|
|Edelbrock choke housing mounts to the carburetor in the same manner as the old choke housing via three screws supplied in the kit. Piston linkage from the kit bolts to the pivot shaft from the AFB indexed by the flats on the end of the shaft.|
|Baffle separates the spring housing from the piston area. The actuating arm on the piston linkage projects through the slot in the baffle. The baffle is located in the housing by the button-shaped recess in the bottom of the baffle.|
|The gasket and spring housing are held in place with three screws and butterfly clips around the raised ridge on the cover. Black ground wire supplied in the kit plugs onto the negative tab on the housing and is grounded at the cover attachment screw.|
|The red wire supplied in the kit plugs onto the positive terminal of the choke cover and connects to a 12-volt terminal on the ignition side of the ignition switch supplying continuous power while the engine is running.|
|Choke butterfly is located on a shaft at the top of the carburetor inlet. It pivots off center which assists in holding it open as air flows through the inlet into the engine.|
|Wiring on the finished conversion will be dressed and included in the engine harness running alongside the rocker cover on its way forward to the alternator.|