One of the mandatory automotive jokes is that "carburetor" is French for "don't touch." Funny thing—the derivation of "carburetor" is actually French, from the word carburer, meaning "to combine with carbon" or, idiomatically, "to tune." The etymology is informative. Gas needs oxygen to burn or explode. To get a controlled explosion, the lifeblood of internal combustion, you need to combine oxygen and atomized gasoline in the right ratio. These days, this function is performed by computer-controlled fuel-injection systems. But for generations, the carb was the only game in town.
Everything a carburetor does is tied to how much air passes through it, so it makes sense to start where the air comes in. At the top of a carburetor is a plate that rotates open and closed. This is the choke. It helps vary the air-fuel mixture for cold starts. Back in the day, when your engine was cold, you'd pull the choke cable to "choke" it, or restrict air flow, producing the fuel-rich mixture needed for starting. Manual chokes gave way to automatic ones that often didn't close correctly, requiring you to pop off the air cleaner and rotate the thing shut to start the engine.
Below the choke is the business end of the carburetor. This is the throat, or barrel, which necks down in the middle to take advantage of Bernoulli's principle: The faster air moves, the lower its pressure, and therefore the easier it is to mix with fuel. Just as you don't see how the warping of space-time creates gravity, you can't watch Bernoulli's principle in action, but the barrel's shape causes the airflow to draw fuel through the carburetor's jets (calibrated holes that meter gasoline) and into the airstream. The engine vacuum sucks in the air-fuel mixture, with airflow regulated in part by a throttle plate at the bottom of the throat.
There's also a fuel reservoir, or float bowl, that serves as the waiting room for the gasoline. More complex carburetors have accelerator pumps to squirt in extra fuel when you nail it and complex internal passages that manage the tricky transition from idle to full throttle.
As for that "don't touch" thing? It depends. There's a Yogi Berra-esque saying that 90 percent of your fuel problems are in your ignition system, so make sure that's healthy and dialed in. But the basic adjustments on any carb are simple: choke, engine idle speed, and fuel mixture at idle. All of these are usually easy to adjust, and the average shop manual will tell you how.
But you want performance, right? First, check to make sure the carb is in good shape. Carburetors wear and grow sloppy with use—old gas turns to varnish, clogging jets, and moving parts like throttle shafts wear or seize. When in doubt, have an expert evaluate what you have. Trying to rebuild a tired carburetor can be an exercise in misplaced loyalty, and replacement is often a better option.
Even with a new carb, if you've hopped up your engine, you'll need to play with the jets. To get it right, you need to know what the engine is doing. The old-school method of jet tuning is to drive the car in a manner that isolates a given jet (small throttle openings for the idle jet, bursts of full throttle for the accelerator pump, high rpm for the main-jet circuit). Then you stop and inspect a spark-plug electrode to judge fuel mixture: ashy gray is too lean, black and sooty too rich.
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This is inaccurate and a pain, so don't bother. The modern answer is to do it by feel—which takes a well-calibrated, experienced butt—or use an aftermarket air-fuel gauge and oxygen sensor. This stuff is rife with complexity, but the general rule is, too rich (too much fuel), decrease the jet size. Too lean (too little fuel), increase it.
Lastly, the multiple carbs on tricked-out engines require synchronization. This yields erotic results when correct, insanity and depravity otherwise. Synchronization on multiple new or recently rebuilt carbs is easy with the right tools, most of which are widely available. On old and worn units, it takes a good ear, experience, intuition, luck, and black magic.
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And, you know, I actually kind of like it when the automatic choke on my '73 BMW 2002 sticks open and I have to pop the air cleaner and rotate it closed. It's like we're an old married couple sharing an in-joke.
A hypothetical downdraft carburetor
- The choke stays open like so during normal running, providing air.
- The throat constricts here. Bernoulli and physics make it easier for fuel to mix with air.
- Air-fuel mixture then travels past the throttle butterfly to the cylinder, and combustion can begin.
- Fuel is collected in the float bowl, from which it travels ...
- ... through the jet and into the airstream.
Understand the basics of air-fuel management, get a feel for how all engines work.
Randomly tinker with carb settings because your engine doesn't run right.
Have someone with a simple, working carburetor show you the ropes in person.
Rob Siegel is a writer and Renaissance wrench from Boston. His book, , is available from Bentley Publishers.