A secondary throttle valve lockout feature is used on many Quadrajet models to prevent opening of the secondary throttle valves during cold operation when wide open accelerations could cause possible engine damage or excessive wear (Figure 27).  On these applications, a lockout lever, located on the float bowl, is weighted so that a tang on the lower end of the lever catches a lock pin on the secondary throttle shaft and keeps 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 choke coil pulls the intermediate choke lever completely down and the fast idle cam drops down so that the cam follower is completely off the steps of the cam.  As the cam drops the last few degrees, it strikes the secondary lockout lever and pushes it away from the secondary valve lockout pin.  This allows the secondary throttle valves to open and operate as described under the Power System.

AIR VALVE LOCKOUT

Some Quadrajet carburetors incorporate an air valve lockout feature (Figure 28), instead of the secondary throttle valve lockout, whereby the air valves are locked closed until the engine is thoroughly warm and the choke valve is wide open.  An air valve lockout lever, mounted on the air horn, is so weighted that a tang on the lever catches the upper edge of the air valve and keeps the air valves closed when the choke valve is closed.  When the thermostatic coil warms up it moves the choke valve toward the open position, the end of the choke rod moves upward and strikes a tang on the air valve lockout lever.  As the choke rod moves up to the end of its travel, it pushes the lockout tang upward and unlocks the air valve.

Also, some early model 4MV carburetors use a vacuum break modulating spring and split choke spring (Figure 28). The vacuum break modulating spring allows the vacuum break (choke valve position) to vary according to ambient temperature.  The vacuum break modulating spring, connected to the vacuum break link, allows varying choke openings depending on the closing force of the thermostatic coil.  As the closing force of the coil increases (cool weather), the link is allowed to move in the slotted lever until the modulating spring overcomes the coil force, or the link is in the end of the slot.  This results in less vacuum break during cooler weather and more vacuum break during warmer weather.

The split choke feature operates during the last few degrees of choke thermostat rotation.  The purpose is to maintain the fast idle speed long enough to keep the engine from stalling, but allow the use of a choke coil which lets the choke valve open quickly. The operation of the split choke feature is controlled by a torsion spring on the intermediate choke lever shaft.  As explained earlier, air pressure action on the offset choke valve tends to force the choke valve open against tension of the choke thermostatic coil.  In the last few degrees of thermostatic coil opening motion, a tang on the intermediate choke lever contacts the end of the torsion spring. This keeps the fast idle cam follower lever on the last step of the fast idle cam longer to maintain fast idle until the engine is thoroughly warm.  The spring works against the thermostatic coil until the coil is hot enough to pull on the intermediate choke lever and overcome the torsion spring tension. The torsion spring must be placed in the specified notch in the vacuum break mounting bracket for application used.

On all 4MV models, the choke system is equipped with an unloader mechanism 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. A tang on a lever 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.

CHOKE SYSTEM WITH SPRING ASSIST CHOKE CLOSING SYSTEM

Some 4MV carburetors use a spring assist choke closing system (Figure 29).  The assist spring is of the torsion type and is added to the intermediate choke shaft.  It exerts pressure on the vacuum break lever to force the choke valve toward the closed choke position. The tension of the torsion spring is overcome by the choke thermostatic coil located on the engine manifold which, during the engine warm up period, will pull the choke valve open.  The addition of a torsion spring assists in closing the choke valve to ensure good engine starting when the engine is cold.

Along with the choke closing assist spring, certain 4MV models use the fast idle cam “pull-off” feature.

When the engine starts and is running, manifold vacuum is applied to the vacuum break diaphragm and the diaphragm plunger moves slowly inward to open the choke valve.  As this happens, a tang on the plunger contacts the end or “tail” of the fast idle cam to “pull-off” the cam from the high step to the lower second step setting.

A slight change in the method of vacuum break adjustment is required on these models that use the fast idle cam “pull-off” feature. (See Adjustment Procedures in the 9D-5 Section of the Delco Carburetor Parts and Service Manual 9X).