What would fuel injection be without a computer controlling all its functions? Less powerful? Sure. Less efficient? Certainly. A heck of a lot more popular? We think so. We'd venture to say that most gearheads, ourselves included, are naturally wary of trusting a mysterious "black box" to control fuel and spark inside a several thousand dollar engine. Carburetors and distributors at least look the part. A computer controller doesn't have the familiar dichromate finish of a Holley 4150. You can't blip the throttle and check the pump shot. There are no jets or metering rods to play with. Heck, most ECUs aren't even mounted in the engine compartment.
But we have a theory about modern technology. It's almost always developed to simplify your life, if you're open-minded enough to embrace it. Thinking about all the different scenarios your engine sees-idling in traffic, wide-open at six-grand through the traps, cruising at 75 in the fast lane-it's an engineering marvel that a carb works at all. Most out-of-the-box carbs will usually work well enough to putt around town before any tuning, but if you're chasing down every last 1/100 second of e.t., you'll have a pretty big chunk of time and frustration invested in perfecting every last passageway in that carb. Makes you wish you could just tell that carb how much fuel you wanted, when you wanted it. And while you're at it, just tell the weights and springs in that distributor to take out a few degrees when the boost comes on.
Common sense suggests that driving your car on the street requires a more versatile fuel system than you'd need to run a quarter-mile down the track. Besides continuously metering air and fuel, a street induction system must provide decent idle quality and fuel enrichment on demand, and be able to fire and run the engine on a cold winter morning. Most street carburetors can meet these basic needs using separate subsystems-internal enrichment and idle circuits, and a choke. It's hard to believe, but EFI systems are less complex and don't need these extra additions to provide the same (or better) level of driveability. Even better, most EFI kits control ignition timing electronically, so you can add or subtract a few degrees of advance to run safely on the edge of detonation. The reasons stem from the most basic principles of their operation.
How A Carb WorksA carburetor takes advantage of the venturi effect to meter a certain fixed ratio of fuel and air into the engine. A sharp "step" in each throttle bore accelerates the incoming air stream, increasing its velocity and creating a region of lower pressure under the fuel booster. Often an additional low-pressure area is created in a venturi-shaped booster. These combined pressure drops draw small droplets of emulsified fuel from small orifices (discharge nozzles) inside the boosters and into the air stream. The size of the main jets controls the amount of fuel that's drawn out of the discharge nozzles. Dip into the throttle a bit more, and a bit more fuel gets drawn from the discharge nozzles, into the air stream, and down the intake runners, where the mixture tumbles and mixes together until, ideally, everything is well mixed, or atomized. Then it's drawn into the cylinders, compressed, and fired.