Float System
Fig. 106. Both the primary and secondary floats function identically and in the same manner as outlined for Model 2GC carburetor. However, a passage in the float bowl, slightly above the nor mal fuel level, connects the primary and secon dary float bowls. In this way, any abnormal rise in level on one side will be absorbed by the other without disrupting engine operation.
![Fig. 106 Rochester_thumb[1]](http://machineryequipmentonline.com/electric-equipment/wp-content/uploads/2020/06/Fig.-106-Rochester_thumb1.jpg "Fig. 106 Rochester_thumb[1]")
Both sides of the carburetor are externally and internally vented to allow even pressure of fuel and air at all times and to allow the escape of fuel vapors during hot idle operation.
To aid in the venting of the carburetor bowl, an idle vent valve and spring assembly is installed in the bowl cover and air horn assembly, Fig. 107. The function of this valve assembly is to improve idle when the engine is warm by venting fumes outside the carburetor rather than into the air cleaner area.
Idle System
Fig. 107. An adjustable idle system on the pri mary side only on some units and on both sides on others. On units having the secondary side inoper ative the idle passages are blocked by gaskets. Except for this note the idle system functions the same as described for the 2GC carburetor.
![Fig. 107 Rochester_thumb[1]](http://machineryequipmentonline.com/electric-equipment/wp-content/uploads/2020/06/Fig.-107-Rochester_thumb1.jpg "Fig. 107 Rochester_thumb[1]")
Part Throttle System
Fig. 108. This system also functions in the same manner outlined for Model 2GC carbur etor. On some units the primary side functions alone up to about 40° of throttle opening, whereas on other carburetors some fuel is used on the secondary side.
![Fig. 108 Rochester_thumb[1]](http://machineryequipmentonline.com/electric-equipment/wp-content/uploads/2020/06/Fig.-108-Rochester_thumb1.jpg "Fig. 108 Rochester_thumb[1]")
Power System
Fig. 109. As the primary throttle valves are opened past 40 degrees, mechanical linkage be tween the secondary and primary throttle valves start to open the secondary valves. The ratio of motion is such that by the time the primary valves have reached wide open position, the secondary valves are also wide open. With all throttle valves wide open, the venturi systems in both sides feed fuel-air mixture through their respective main metering systems.
A pair of spring-loaded, air velocity operated, auxiliary throttle valves are located in the secon dary bores above the regular throttle valve s. When the throttle valves are moved to their wide open position and engine speed is low, there is sufficient air flow through the secondary bores to force the auxiliary valves to open. This will con centrate all air flow through the primary throttle bores with better metering of fuel and air. In this condition the carburetor is functioning as a two barrel unit. As engine speed increases, the force of the air acting on the auxiliary valves increases to the point where the auxiliary valves are forced to open.
In addition, fuel How is supplemented through a vacuum-controlled power valve on the primary side. This valve functions in a manner similar to that described for Model2GC carburetor.
![Fig. 109 Rochester_thumb[1]](http://machineryequipmentonline.com/electric-equipment/wp-content/uploads/2020/06/Fig.-109-Rochester_thumb1.jpg "Fig. 109 Rochester_thumb[1]")
Accelerating Pump System
Fig. 110. Since the secondary throttle valves remain closed during part throttle operation, only the primary side needs the extra boost of fuel for acceleration. Hence the primary side only contains the pump system. It functions in the same manner as Model2GC carburetor.
![Fig. 110 Rochester 4GC accelerating_thumb[1]](http://machineryequipmentonline.com/electric-equipment/wp-content/uploads/2020/06/Fig.-110-Rochester-4GC-accelerating_thumb1.jpg "Fig. 110 Rochester 4GC accelerating_thumb[1]")
Choke System
Fig. 111. Since the secondary throttle valves remain closed for idle and part throttle operation, only the primary side requires a choke system.
![Fig. Ill Rochester 4GC choke system_thumb[1]](http://machineryequipmentonline.com/electric-equipment/wp-content/uploads/2020/06/Fig.-Ill-Rochester-4GC-choke-system_thumb1.jpg "Fig. Ill Rochester 4GC choke system_thumb[1]")
When the choke is closed , the fast idle earn is raised. The r aised position of the fast idle cam “locks out” any opening of the secondary throttle valves by means of a lockout lever, which is free to m ove only when the cam is fully lowered.
Model 4MV, 4MC
The “Quadrajet” carburetor, Fig. 112, is a 4-bar rel unit with versatility and prin ciples of operation that make it adaptable f or small to large engines without design changes.
![Fig. 112 Rochester Model 4MV carburetor_thumb[1]](http://machineryequipmentonline.com/electric-equipment/wp-content/uploads/2020/06/Fig.-112-Rochester-Model-4MV-carburetor_thumb1.jpg "Fig. 112 Rochester Model 4MV carburetor_thumb[1]")
The fuel bowl is centrally located to avoid problems of fuel slosh, causing engine turn cut out, and delayed fuel How to the carburetor bores.
The float needle valve is pressure balanced to permit use of a single small float.
The primary side has small bores and a triple venturi for fine fuel control in the idle and econ omy ranges. The secondary side has large bores and an air valve for high air capacity.
A hot idle compensator, consisting of a bi-metal strip, a valve and a mounting bracket located at the rear of the carburetor adjacent to the secon dary bores supplies additional air to the idle mix ture during prolonged hot idle periods.
The primary side has six systems of operation; float, idle, main metering, power, pump and choke. The secondary side has one metering sys tem which supplements the primary main meter ing system and receives fuel from a common float chamber.
Float System
Fig. 113. Fuel enters the float chamber at the top to prevent incoming fuel vapors from mixing with solid fuel in the bottom of the float bowl. A plastic filler block is located in the top of the float chamber just above the float valve. This block prevents fuel slosh on severe braking and main tains a more constant level to prevent stalling.
The float chamber is internally and externally vented to allow an even pressure of fuel and air at all times and to permit fuel vapors to escape during hot engine operation. The external vent prevents rough engine idle and long hot engine starting.
![Fig. 113 Rochester Models 4MV,_thumb[1]](http://machineryequipmentonline.com/electric-equipment/wp-content/uploads/2020/06/Fig.-113-Rochester-Models-4MV_thumb1.jpg "Fig. 113 Rochester Models 4MV,_thumb[1]")
Idle System
Fig. 114. A hot idle compensator valve is used which permits more air to enter the manifold during hot engine operation. This offsets the richer mixtures and maintains a smoother idle. Except for this the idle system operates the same as outlined previously for the 2GC carburetor.
![Fig. 114 Rochester Models 4MV,_thumb[1]](http://machineryequipmentonline.com/electric-equipment/wp-content/uploads/2020/06/Fig.-114-Rochester-Models-4MV_thumb1.jpg "Fig. 114 Rochester Models 4MV,_thumb[1]")
Main Metering System
Fig. 115. This system consists of main metering jets, vacuum operated metering rods, main fuel well, main well air bleeds, discharge nozzles and triple venturi. The system operates as follows:
During cruising speeds and light engine loads, manifold vacuum is high and holds the metering rods down in the main metering jets against spring tension. Fuel flow from the float bowl is metered between the metering rods and the main jet orifice.
![Fig. 115 Rochester Models 4MV,_thumb[1]](http://machineryequipmentonline.com/electric-equipment/wp-content/uploads/2020/06/Fig.-115-Rochester-Models-4MV_thumb1.jpg "Fig. 115 Rochester Models 4MV,_thumb[1]")
As the primary throttles are opened beyond the off-idle range allowing more air to enter the engine manifold, air velocity increases in the ven turi. This causes a drop in pressure in the large venturi which is increased many times in the double boost venturi. Since the low pressure (vacuum) is now in the smallest boost venturi, fuel flows from the main nozzles as follows :
Fuel flows from the float bowl through the main metering jets into the main fuel well and is bled with air from the vent at the top of the main well and side bleeds. The fuel in the main well is mixed with air from the main well air bleeds and then passes through the main discharge nozzle into the boost venturi. At the boost venturi the mixture then combines with the air entering the engine through the carburetor and passes through the intake manifold and on into the cylinders.
Power System
Fig. 116. On part throttle and crmsmg ranges, manifold vacuums are sufficient to hold the power piston down against spring tension so that the larger diameter of the metering rod tip is held in the main metering jet. Mixture enrichment is needed at this point. However, as engine load is increased to a point where mixture enrichment is required, the spring tension overcomes the vacuum pull on the power piston and the tapered primary metering rod tip moves upward in the main metering jet. The smaller diameter of the rod tip allows more fuel to pass through the main metering jet and enrich the mixture flowing into the primary main wells and out the main dis charge nozzles. As engine speed continues to in crease, the primary side can no longer meet the engine requirements and then the secondary side is used as follows:
When the engine reaches a point where primary bores cannot meet engine demands, the primary throttle lever through connecting linkage to the secondary throttle shaft lever, begins to open the secondary throttle valves. Air flowing through these bores creates a low pressure (vacuum) be neath the air valve, atmospheric pressure on top of the air valve forces the valve open against spring tension. This allows the required air for increased engine speed to flow past the air valve.
When the air valve begins to open, the upper edge of the valve passes the accelerating well port. The port is then exposed to manifold vacuum and will start to feed fuel from the accelerating wells.
The secondary main discharge nozzles are lo cated below the air valve and above the secondary throttle plates. Being in the area of lowest pres sure, they begin to feed fuel as follows:
When the air valve begins to open it rotates a plastic cam attached to the main air valve shaft. The cam pushes on a lever attached to the secon dary main metering rods. The cam pushes the lever up, raising the metering rods out of the secondary orifice plates. Fuel flows from the float chamber through the secondary orifice plates into the secondary main wells, where it is mixed with air from the main well tubes. The mixture next travels from the main wells to the secondary dis charge nozzles and into the secondary bores. Here the fuel is mixed with air traveling through the secondary bores to supplement the mixture de livered from the primary bores, and then goes on into the engine.
![Fig. 116 Rochester Models 4MV,_thumb[1]](http://machineryequipmentonline.com/electric-equipment/wp-content/uploads/2020/06/Fig.-116-Rochester-Models-4MV_thumb1.jpg "Fig. 116 Rochester Models 4MV,_thumb[1]")
Air Valve Dashpot
Fig. 117. The secondary air valve has an at tached piston assembly which acts as a dampener to prevent oscillation of the valve due to engine pulsations. The dampener piston operates in a well which is filled with fuel from the float bowl. The motion of the piston is retarded by fuel which must by-pass the piston when it moves up in the fuel well. The piston is loosely attached to a plunger rod. The rod has a rubber seal which retains the dampener piston to the plunger rod and also acts as a valve. The purpose of the valve is to seat on the piston when the air valve opens and the piston rod moves upward. This closes off the area through the center of the piston and slows down the air valve opening to prevent secondary discharge nozzle lag.
![Fig. 117 Rochester Models 4MV,_thumb[1]](http://machineryequipmentonline.com/electric-equipment/wp-content/uploads/2020/06/Fig.-117-Rochester-Models-4MV_thumb1.jpg "Fig. 117 Rochester Models 4MV,_thumb[1]")
Accelerating Pump System
Fig. 118. This system is located on the primary side of the carburetor. It consists of a spring loaded plunger and return spring operating in a fuel well. The plunger is operated by a lever on the air horn which is connected to the throttle lever by a rod.
![Fig. 118 Rochester Models 4MV,_thumb[1]](http://machineryequipmentonline.com/electric-equipment/wp-content/uploads/2020/06/Fig.-118-Rochester-Models-4MV_thumb1.jpg "Fig. 118 Rochester Models 4MV,_thumb[1]")
During throttle closing the plunger moves up in the well and allows fuel to enter the well through a slot in the top of the well. It flows past the cup seal into the bottom of the well.
When the throttle valves are opened the linkage forces the plunger down. The pump cup seats instantly and fuel is forced through the discharge passage where it unseats the pump discharge check ball and passes on to the pump jets where it sprays into the venturi area.
The pump discharge check ball seats in the pump discharge passage during upward motion of the plunger so that air will not be drawn in; otherwise a momentary acceleration lag could result.
During high speed operation, a vacuum exists at the pump jets. A cavity just beyond the jets is vented to the top of the air hom, outside the bores. This acts as a suction breaker so that when the pump is not in operation fuel will not be pulled out of the pump jets into the venturi area. This insures a full pump stream when needed and prevents any fuel “pull over” from the pump discharge passage.
4MV Choke System
A thermostatic coil is located in the engine manifold and is calibrated to hold the choke valve closed when the engine is cold. A vacuum break is used to overcome the thermostatic coil and open the choke valve to a point where the engine will run without loading or stalling. ·
4MC Choke System
The system in this model carburetor is basically the same as the one discussed for the 4MV type except that the thermostatic coil is mounted on the carburetor.