The Rochester Quadrajet is...
The Rochester Quadrajet is a complex, highly efficient carburetor. Rebuilt and staged properly, this carburetor design is far more tolerant of altitude changes and provides excellent driveability. The Q-jet served on engines ranging from Buick's V-6 to muscle cars and Cadillac's 500ci V-8!
Gasoline is volatile when vaporized with oxygen. As a motor fuel, gasoline burns best at a ratio of 14.7:1-roughly 14.7 pounds of air to each pound of gasoline. This optimal burn, known as "stoichiometric," serves best when the vehicle is at cruise or similar loads. For more power, like passing, accelerating, or moving a heavy load, a richer air/fuel mixture becomes necessary. Under lighter loads, a leaner mixture may suffice and provides improved fuel economy.
In order to "mix" the air and fuel before combustion, the earliest internal combustion engines had carburetors. Although designs changed, the principle of the carburetor has remained the same, as a means for mixing air and fuel in correct proportions for a given engine load. Carburetors, refined over time, served quite well until the advent of electronic fuel injection.
Carburetor Circuits
Engine speed, load, and starting demands each dictate how fuel will be delivered. When the throttle is closed or slightly open, manifold vacuum is high, and fuel can flow by means of engine vacuum. If the throttle opens wider or heavy throttle is demanded, manifold vacuum drops accordingly. A carburetor must flow even more fuel at lower manifold vacuum-the greater the engine load, the lower the manifold vacuum.
To deliver fuel at lower manifold vacuum and wider throttle openings, carburetors maintain fuel flow volume by the venturi effect. The throats of the carburetor have sized-down sections. As a column of air moves through the carburetor's throat, reduction in the bore size raises the velocity (speed) of the air column. This creates a low-pressure effect in the venturi areas. The vents above the float chamber enable fuel to move through the main discharge tubes into the low-pressure area of the venturi. Once discharged, the air/fuel mixture continues down the throttle bores and into the engine.
Stripped for rebuilding, the...
Stripped for rebuilding, the pieces shown require caustic cleaning, thorough rinsing, and compressed air drying. When rebuilding a carburetor, begin with a complete inventory of the parts. Match up identification numbers whenever the carburetor's origins are uncertain. Jets and metering rods have engine-specific sizing and must match your engine's displacement and requirements.
The carburetor must operate over a wide range of throttle openings, airflow characteristics, and manifold vacuum. Distinct "circuits" function in the different operating modes. Modern carburetors have: 1) an idle system, 2) off-idle system, 3) main metering system, 4) power system, 5) accelerating pump system, and 6) the cold-start choke system. Other refinements might include a secondary air valve on a four-barrel carburetor or a fully vacuum controlled secondary throttle. (A vacuum dashpot can control the opening of the secondary air valve.) When the throttle valves open wide, low manifold vacuum signals the air valve to open.
On a carburetor like the Quadrajet, the idle system picks up fuel from the main fuel well. Sized idle tubes draw fuel into discharge tubes that mix this fuel with air from the idle air bleeds and the off-idle discharge ports. The mixture enters the carburetor bores via the adjustable idle needle orifices. These idle needle screws control the mixture flow at an idle and off-idle throttle position.
The off-idle system still relies on manifold vacuum. As the throttle valves begin to open, strong manifold vacuum pulls fuel through the off-idle port. This port is just above each throttle valve and gets a strong vacuum signal as the valve begins to open. Note: This is the vacuum that we call ported. Ported vacuum signals are the typical source for a distributor's vacuum advance canister. Like the vacuum advance, the off-idle system responds with initial tip-in of the throttle.
 By comparison, this muscle...  By comparison, this muscle era Carter AFB has removable venturi, an attached choke assembly and dual floats. The AFB (aluminum four-barrel) main body includes the throttle valves. Quadrajets have separate castings for the throttle valve plate, main body, and top cover or air horn. Holley's square-flange, four-barrel carburetors, like the 4150/4160 series, use end floats. |  This Quadrajet throttle plate...  This Quadrajet throttle plate shows its unique "spread bore" design. The primary valves are considerably smaller than the secondaries. These carburetors deliver exceptional fuel efficiency when not pressed to use the secondary bores. Despite the Quadrajet's many advantages, G.M. and others opted for Holley square-flange carburetors on their hottest single four-barrel engines. |  The four small tubes shown...  The four small tubes shown are the secondary accelerating well tubes (outer pair) and the secondary high-speed air bleeds. These tubes must be clear of any obstructions or debris. They are sized and should be cleaned carefully without increasing the diameter of orifices. A caustic soak, clean water rinse, and compressed air drying should suffice here. |
 A new needle and seat assembly...  A new needle and seat assembly is always part of an overhaul. Use a large screwdriver blade that will fit the slot snugly. This is soft brass, which will slough if chipped. Do not distort brass parts and be certain the seat is secure with a new gasket. Make sure small orifices are clear by air blowing passageways during assembly. |  Needle and seat in place,...  Needle and seat in place, the brass float should be checked for free movement. The float height and drop are critical measurements. They determine the level of fuel in the bowl. A high float setting causes flooding. Low float height will create fuel starvation and lean misfire under high fuel demands. Follow the assembly instructions and note whether the gasket should be in place during measurements. |  On the Quadrajet, float height...  On the Quadrajet, float height requires holding the pivot pin in position. Gently push on the back edge of the float to seat the needle and raise the float to full height. Use a quality, precisely marked gauge to measure the float height. On many carburetors, float drop is an equally important measurement. Correct drop unseats the needle without dislodging it. |