There's much to be said for technology-it certainly has had an impact on the performance of new cars and trucks. But there's also a fair amount to be said for simplicity. Just because something is simple doesn't mean it's not effective. That certainly holds true for carburetors. While electronic fuel injection relies on a variety of sensors and a computer to deliver the appropriate amount of fuel to the engine, carburetors simply rely on vacuum. Basically, all carburetors-single-, dual-, or four-barrel-work the same way and have five distinct systems.
FloatThe float maintains a constant level of fuel in the float bowl, and while it seems to be a simple system, it has to function correctly to maintain the proper air/fuel ratio. If the fuel level is too high, the engine will run rich and may even flood. Too low and the engine may run lean.
Main SystemWhen the engine is running, it draws air down through a restricted area called a venturi, and a low-pressure area is created while the fuel in the float chamber is exposed to atmospheric pressure by a vent. As there is more pressure in the float chamber than the venturi, fuel flows through a restriction called a jet and is discharged into the airstream. The airflow through the venturi is controlled by the position of the throttle valves (as well as engine speed); the maximum amount of air the carburetor can pass is its cfm (cubic feet per minute) rating. But while all carburetors are rated in cfm, comparisons can be difficult because the standards for testing are different. Four-barrels are tested at 1 1/2 inches of mercury; single- and two-barrels are tested at 3 inches of mercury.
Idle SystemAt idle and very low speeds the throttle valves are not open far enough to create a vacuum in the venturi. As a result, no fuel is discharged. However, there is vacuum below the throttle valves, so to supply the fuel the engine needs at idle, there are fuel passages below the throttle valves called idle ports. The idle mixture screws control the amount of fuel that is discharged. As the throttle valves are opened, the idle ports are no longer exposed to vacuum, and the fuel supply stops.
As a part of the idle system, most carburetors also have transfer ports, small slots located just above the idle ports that supply fuel as the transition is made from the idle to the main system.