Carburetor Tuning Basics - Night School

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.

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.