This illustration is for a 1981 Chevy truck application. This particular carburetor uses both a primary, or front choke pull-off and a secondary, or rear choke pull-off. Starting in the mid 70's, Quadrajet starting using this type of choke system and many of it's 4 barrel carburetors.
The primary choke pull-off is tested by applying 15 lbs of vacuum to the inlet tube. The arm should pull in and stay there until vacuum is released.
The secondary choke pull-off will require 20 lbs of vacuum. You will also need to plug the relief hole (on the pulloff itself) if one is present. Same as above, the arm should pull in but will be slow moving. The secondary pulloff helps keep the secondary air valve open a bit so that the engine doesn't run too rich when cold.
The particular Quadrajet uses an integral (mounted on the carburetor), choke thermostat.
Install the thermostat with a gasket, so that the tab D is up against the lever A. Twist the thermostat until the choke valve closes. You may have to hold the throttle open a bit so that it isn't holding the choke open. When the choke closes, twist the thermostat another 1/8 to put a slight load on the valve.
As the thermostat heats up the thermostat coil will expand allowing the choke valve to open.
To test get the engine to operation temperature. The choke valve should be fully open.
B - This screw holds the choke housing to the carburetor.
C - The retainer screws are sometimes riveted originally. Drill out the rivets and tap the holes, or use self tapping screws.
The Quadrajet choke valve is located in the primary side of the carburetor. It provides the correct air/fuel mixture enrichment to the engine for quick cold engine starting and during the warm-up period. The air valve is locked closed until the engine is thoroughly warm and choke valve is wide open.
The choke system consists of achoke valve located in the primary air horn bore, avacuum diaphragm unit, fast idle cam, connecting linkage, air valve or secondary throttle valve lockout lever and athermostatic coil. The thermostatic coil is located in the engine manifold and is connected by arod to the intermediate choke shaft and lever assembly. Choke oper¬ ation is controlled by the combination of intake manifold vacuum, the off-set choke valve, temperature, and throttle position.
The thermostatic coil located in the engine manifold is calibrated to hold the choke valve closed when the engine is cold.
NOTE: To close the choke valve, the primary throttle valves have to be opened to allow the fast idle cam follower to by-pass the steps on the fast idle cam and come to rest on the highest step of the fast idle cam. When the choke valve is closed, the air valve lockout lever is weighted so that atang on the lever catches the upper edge of the air valve and keeps the air valve closed.
During engine cranking, the choke valve is held closed by the tension of the thermostatic coil. This restricts air flow through the carburetor to provide aricher starting mixture. Some late models use acranking enrichment system. Two fuel feed holes located just beneath the choke valve supply added fuel for cold enrichment during the cranking period. The extra fuel is supplied through channels which lend to the secondary well bleed tubes and allow fuel to be drawn from the secondary main wells.
When the engine starts and is running, manifold vacuum applied to the vacuum diaphragm unit mounted on the float bowl opens the choke valve to apoint where the engine will run without loading or stalling. Also at this point, the cold enrichment feed holes are no longer in alow pressure area so they cease to feed fuel. From this point on they will be used as secondary main well air bleeds. At the same time, the fast idle cam follower lever on the end of the primary throttle shaft will drop from the highest step on the fast idle cam to a lower step when the throttle is opened. This gives the engine sufficient fast idle and correct fuel mixture for running until the engine begins to warm up and heat the thermostatic coil. As the thermostatic coil on the engine manifold becomes heated, it relaxes its tension and allows the choke valve to open further because of intake air pushing on the off-set choke valve. Choke valve opening continues until the ther¬ mostatic coil is completely relaxed at which point the choke valve is wide open.
When the engine is thoroughly warm, the choke coil pulls the intermediate choke lever completely down and allows the fast idle cam to rotate so that the cam follower drops off the last step of the fast idle cam allowing the engine to run at normal speeds. When the choke rod moves upward in the choke shaft lever, the end of the rod strikes atang on the air valve lockout lever. As the rod moves to the end of its travel, it pushes the lockout tang upward and unlocks the air valve.
The choke system is equipped with an unloader mech¬ anism which is designed to partially open the choke valve, should the engine become loaded or flooded. To unload the engine, the accelerator pedal must be depressed so that the throttle valves are held wide open. Atang on alever on the choke side of the primary throttle shaft contacts the fast idle cam and through the intermediate choke shaft forces the choke valve slightly open. This allows extra air to enter the carburetor bores and pass on into the engine manifold and cylinders to lean out the fuel mixture so that the engine will start.
On some models, asecondary throttle valve lock-out used in place of the air valve lock-out. The type design isN^ used on apphcations where little or no air flow can be tolera¬ ted from the secondary throttle bores during engine warm up. On these applications, alock-out lever located on the float bowl is weighted so that atang on the lower end of the lever catches alock pin on the secondary throttle shaft and holds the secondary throttle valves closed. As the engine warms up, the choke valve opens and the fast idle cam drops. When the engine is thoroughly warm, the choke valve is wide open and the fast idle cam drops down so the cam follower is “ completely off the steps of the cam. As the cam drops the last few degrees, it strikes the secondary lock-out lever and pushes it away from the secondary valve lock-out pin. This allows the secondary valves to open and operate as described under the power system.
The Model 4MC choke system consists of achoke valve located in the primary air horn bore, achoke housing and vacuum diaphragm assembly, fast idle cam, connecting linkage, air valve lockout lever, and thermostatic coil. Choke operation is controlled by the combination of intake manifold vacuum, the offset choke valve, temperature, and throttle position.
The thermostatic cod is calibrated to hold the choke valve closed when the engine is cold. NOTE; To close the choke valve, the primary throttle valves have to be partially opened to allow the fast idle cam follower to by-pass the steps on the fast idle cam and come to rest on the highest step of the fast idle cam.
When the choke valve is closed, the air valve lockout lever is weighted so that atang on the lever catches the upper edge of the air valve and keeps the air valve closed.
During engine cranking, the choke valve is held closed by tension of the thermostatic cod. This restricts air flow through the carburetor to provide the richer starting mixture required. When the engine starts and is running, manifold vacuum apphed to the vacuum diaphragm mounted on the choke housing forces the choke valve open to apoint where the engine will run without loading or stalling. This is accomplished through aplastic plunger mounted on the vacuum break diaphragm.
Engine vacuum pulls inward on the diaphragm and the plunger strikes the vacuum break tang inside the choke housing which, in turn, rotates the intermediate choke shaft and through connecting linkage opens the choke valve. At the same time, the fast idle cam follower lever on the end of the primary throttle shaft will drop from the highest step on the fast idle cam to the second step if the throttle is opened. This gives the engine sufficient fast idle and correct fuel mixture for running until the engine begins to warm up and heat the thermostatic coil. As the thermostatic coil becomes heated, it relaxes its tension and allows the choke valve to open further because of intake air pushing on the off-set choke valve. Choke valve opening continues until the thermostatic coil is completely relaxed and the choke valve is wide open. During engine warm-up the choke coil rotates, forcing the intermediate choke shaft and lever clockwise. This allows the fast idle coil to rotate until the cam follower drops off the last step of the fast idle cam so the engine will run at normal idle speeds. When the choke moves toward the open position, the end of the choke rod strikes atang on the air valve lock-out. As the choke rod moves to the end of its travel, it pushes the lock-out tang upward and unlocks the air valve. The choke system is equipped with an unloader mech¬ anism which is designed to partially open the choke valve, should the engine become loaded or flooded during the start¬ ing period. To unload the engine, the accelerator pedal should be depressed so that the throttle valves are held wide open. Atang on alever on the choke side of the primary throttle shaft contacts the fast idle cam and through the intermediate choke shaft forces the choke valve slightly open. This allows extra air to enter the carburetor bores and pass on into the engine manifold and engine cylinders to lean out the fuel mixture so that the engine will start.