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The choke housing on late model 4MC carburetors (Figure 35) is also located on the float bowl, but with a separately mounted vacuum break diaphragm unit. The vacuum diaphragm unit on some 4MC models includes and internal delay valve for good control of choke mixtures during the engine warm-up period (see description of Delayed Vacuum Break System, Figure 31).
CHOKE SYSTEM – M4MC-M4MCA
Similar to 4MC models, an integral choke housing and an exhaust heated air thermostatic coil assembly are mounted on the carburetor float bowl on M4MC-M4MCA models (Figure 36). A single or a dual vacuum break system is used for control of choke mixtures during the warm-up period. A special adjusting screw is included on the front vacuum break unit on all “modified” Quadrajet models. The vacuum break is adjusted by turning this screw to position a tang on the plunger that contacts the vacuum break lever.
On some models, the front and/or rear vacuum break units are delayed in operation by an internal delay valve (see inset, Figure 36). The valve delays opening of the choke valve a few seconds to allow the engine to run on richer mixtures.
A clean air purge feature is used in either the front and rear vacuum break diaphragm units or in the rear unit only, depending upon carburetor application, to prevent dirt from plugging the internal delay valve. A filter element is installed internally with a small bleed hole located in the end cover of the diaphragm unit. During engine operation, vacuum acting upon the diaphragm pulls a small amount of filtered air through the bleed hole in the end cover to purge the system of any fuel vapors or dirt contamination which might be pulled into the internal delay valve located inside the diaphragm unit. During adjustment of the front and rear vacuum breaks, it will be necessary to plug the end cover of the vacuum break unit with tape, making sure to seal the small bleed hole.
Also, a vacuum inlet check ball may be used in the vacuum inlet tube on the front and rear vacuum break units. The purpose of the inlet check ball is to prevent excess dirt and vapor contaminants from plugging the small internal delay valve in the diaphragm unit in case of engine “backfire” or “dieseling” conditions.
The rear vacuum break unit on some applications may include a tension (bucking) spring in the diaphragm plunger head. (See previous explanation Plunger Bucking Spring Operation Page 21).
On some M4MC-M4MCA models, the rear vacuum break unit includes a choke closing assist spring (See Figure 30 and previous explanation of closing assist spring operation Page 20).
Some M4MC-M4MCA models incorporate the “trapped stat” thermostatic coil design whereby the end of the thermostatic coil is installed in a slot in the coil pick-up lever inside the choke housing. In this way, the coil is kept in contact with the pick-up lever at all times for prompt response to choke coil tension.
FAST IDLE CAM PULL-OFF FEATURE
A fast idle cam “pull-off” feature is included on some M4MC-M4MCA carburetor applications. Manifold vacuum to the rear vacuum break diaphragm is supplied through a water temperature controlled thermal vacuum switch (TVS) and by an electrically operated time delay solenoid.
During cold operation when manifold vacuum is applied through the TVS and the solenoid to the rear vacuum break diaphragm, the diaphragm plunger moves inward pulling on the vacuum break rod which rotates the vacuum break lever to “pull-off” the fast idle cam from the high step to the lower step setting. Thus, the cam “pull-off” feature prevents prolonged high idle speeds during the warm-up period.
Figure 37 M4ME-M4MEA Electric Choke (Typical)
CHOKE SYSTEM – M4ME M4MEA WITH ELECTRIC CHOKE
Some “modified” Quadrajet models include an electric choke (M4ME-M4MEA) replacing the conventional hot air choke system (Figure 37). An electrically heated thermostatic coil assembly is installed in the choke housing mounted on the side of the float bowl. In addition, these models include the dual vacuum break system consisting of front and rear vacuum break units.
The rear vacuum break unit may include a plunger “bucking spring” depending on application. Also, some applications include the fast idle cam “pull-off” feature. During cold operation, manifold vacuum is supplied to the front vacuum break diaphragm by a water temperature thermal vacuum switch (TVS). When manifold vacuum is applied to the front vacuum break diaphragm, a tang on the diaphragm plunger contacts a tang on the fast idle cam as the plunger contacts a tang on the fast idle cam as the plunger moves inward to “pull-off” the fast idle cam from the high step to the lower step setting. This action prevents prolonged high idle speeds during engine warm up.
The electric choke is heated by an electric current and a ceramic resistor in the electric choke assembly warms the thermostatic coil for precise timing of choke valve opening for good warm up performance. The electric choke operates as follows:
The electric choke receives an electric current operating through the engine oil pressure switch whenever the engine is running. The electric current flows to a ceramic resistor that is divided into separate sections – a small section for gradual heating of the thermostatic coil, and a large section for rapid heating of the thermostatic coil. Electric current is directed internally to one or both of the resistor sections relative to ambient air temperature conditions to provide choke coil heat as required for good engine starting and driving performance.
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