Other Quadrajet carburetors use a dual delayed vacuum break system consisting of front and rear vacuum break diaphragm units (Figure 33).
The dual delayed vacuum break system operates as follows:
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 a richer starting mixture.
When the engine starts and is running, manifold vacuum is applied to both vacuum break units which are mounted on the side of the float bowl. The front (primary) vacuum break diaphragm opens the choke valve to a point where the engine will run without loading or stalling. As the engine is wetted and friction decreases after start, a delay valve in the rear (auxiliary) vacuum break unit causes a delayed action to gradually open the choke valve a little further until the engine will run at a slightly leaner mixture to prevent loading.
A plunger "bucking spring" may be included on the diaphragm plunger of the rear diaphragm unit (See explanation, plunger bucking spring operation).
A clean air purge feature is added to the rear vacuum break diaphragm unit on some 4MV models using the dual delayed vacuum break system (Figure 33). A clean air bleed, added to the tube at the rear of the rear vacuum break unit and located beneath a rubber covered filter, purges the system of any fuel vapors and dirt which possibly may enter the internal delay valve to disrupt operation. A change in adjustment procedure for setting the rear vacuum break is required on those models using the clean air purge feature. (See Adjustment Procedures, Section 5, of the Delco Carburetor Parts and Service Manual 9X).
CHOKE SYSTEM "4MC (EARLY)
The model 4MC carburetor differs from the 4MV model in that the thermostatic choke coil is located inchoke housing, mounted directly to the side of the carburetor float bowl, instead of a "remote choke" coil mounted on the intake or exhaust manifold.
On 4MC models, engine vacuum is supplied through an orifice in the choke housing which pulls heat from the manifold heat stove into the choke housing and heat gradually relaxes choke coil tension.This allows the choke valve to continue opening through inlet air pressure pushing on the offset choke valve and the weight of the choke linkage pulling the valve open.
On early model 4MC carburetors (Figure 34), the vacuum break diaphragm is mounted integrally with the choke housing located on the float bowl. An adjustable plastic plunger is included as a part of the vacuum break diaphragm. During cold operation after the engine starts and is running, manifold vacuum applied to the vacuum diaphragm pulls the diaphragm inward 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 to the "vacuum break" position.