Everything You Need to Know About Aftermarket Carburetors

While it’s easy to imagine a steady stream of liquid fuel pouring into your intake, garden-hose like, the reality is much different. In truth, carburetors are complex devices that do a complex job: directing fuel and air through a maze of passages, to turn liquid fuel into atomized droplets. Why? Well, liquid fuel doesn’t burn, so fuel must be atomized and vaporized before it combusts—completely vaporized fuel leads to better combustion, bigger power, and lower emissions.

Low-Pressure Fuel “Injection”

Here’s a big-picture look at how that happens: first, the vehicle’s fuel tank, lines, and fuel pump supply fuel to the carburetor’s float bowls. Float bowls are reservoirs that use a float, an arm, and a needle-and-seat assembly to regulate the float height—and the fuel level. When the float goes up, the needle and seat prevent more fuel from pouring in. When the float goes down, the needle and seat allow more fuel in. In doing so, the carb gets a steady diet of petrol. But unlike with fuel injectors, fuel in a carb’d engine isn’t forced in; it’s pulled into the engine by a pressure differential, caused by a phenomenon called the venturi effect. This scientific wizardry involves fluid pressure, flow restriction, and Bernoulli—the scientist, not some Mustang driver from Englishtown.

[Holley Image]

Fuel travels from the bowls through emulsion tubes—small cylinders with tiny holes. Combined with air from air bleeds, these tubes pre-atomize the fuel before it gets to the venturis. Now, let’s talk venturi effect. A venturi uses physics to push fuel through the carb and into the intake manifold. How? It’s shaped like an hourglass: wide on both ends, and necked down in the middle. When fuel hits the first wide end, it has low speed and high pressure. But as it passes through the narrow middle section, the fuel’s speed increases but its pressure decreases. And it’s that pressure difference—about 1 psi lower than the normal, 14.7-psi atmospheric pressure seen at the fuel bowls—that pushes fuel into your engine.

A Circuitous Route

Carburetors contain numerous air and fuel circuits, which mirror the engine’s varying rpm and load situations. Here’s what they do:

The idle circuit provides fuel when the engine is at idle.

The primary circuit provides fuel for the primary side of the carb during light-throttle operation.

The secondary circuit provides additional fuel when the secondaries open during hard acceleration.

The accelerator pump circuit speeds up fuel flow by adding gas during transition from light to heavy throttle—like when the secondaries open and the engine goes lean from the extra airflow. FYI, a carb’s accelerator pump circuit is the only one not affected by airflow.

The fuel enrichment circuit (aka the power valve) is normally located on the primary side, only opens during WOT, and can add up to 10 more jet sizes of fuel on demand. And because it’s held closed by manifold vacuum under non-WOT conditions, smaller primary jets can be used for crisp part-throttle operation, with the power valve providing enough fuel for full-throttle performance, too.

And there’s a bunch of idle-related circuits that are both air-only and air/fuel mixtures. Those include idle air bleeds, idle transfer air bleeds, and idle discharge holes/slots.

Jet Setting

A carburetor’s jets are simply restrictors in varying sizes that determine how much fuel passes through—and what an engine’s air/fuel ratio is. Jets are numbered based on what they flow; jet sizes can sometimes correspond to the jet’s diameter, and sometimes not. But the bigger the jet number, the greater the fuel flow so larger jets richen the mixture, and smaller jets lean it out.

Edelbrock/Carter AFB carbs also use metering rods—Allen-key-shaped devices with stepped ends that extend into the carb’s main jets. Similar to power valves, metering rods are available in various sizes, are actuated by specially tuned springs, and restrict or increase fuel flow through the jets depending on engine load.

Flange Type

Different carburetors are available in different flange types: there’s the Holley 4150-style square-bore, or the QuadraJet spread-bore. Be sure that whatever carb you buy will mate with whatever intake manifold that you have.

Vacuum vs. Mechanical Secondaries

Carburetors use two methods to open up a four-barrel’s secondaries: Vacuum-secondary carbs use engine vacuum in the primary venturis to open the secondaries. As the rpm increases the vacuum increases, and the secondaries open gradually. This type of carb is more street-friendly and gets better gas mileage—though heavy vehicles with tall gears can benefit as well.

Conversely, mechanical-secondary carbs are “hard wired” in; that is, directly actuated by your right foot. They typically open right around half-throttle, hit very hard compared to a vacuum-actuated carb, and trade drivability for a more high-performance feel.

While it’s easy to say “vacuum setups are for street and mechanicals are for race,” the truth is that today’s highly refined carburetors can be set up to work fine in both worlds. You gotta love progress.

Manual Vs. Electric Choke

Your choke options are either manual, which are controlled by a cable, or electric, which are controlled automatically. Manual is more work, what with the pedal pumping and rod-choking and cold-weather frustration and what not, but race-intended mills can use ‘em (or not if you like). And electric is hands-off and easy, and great for street-driven rides. Combined with vacuum secondaries, electric choke makes for a killer daily-driven combo.

Tuning, Tweaking, and Buying Tips

“Leave It Alone” And Other Useful Carb Wisdom

The gearhead mentality means not leaving well enough alone. Seventy years ago, being a hot rodder meant building a performance part literally from scratch, so fiddling with stuff came in handy. But in today’s climate of insane R&D time and precise quality control, that mentality can come back and bite us.

Case in point: buying a new carburetor. Here’s where we mess up:

Don’t overestimate how much carb is needed. Instead of sifting through 100 suggestions from well-meaning gearheads, check out our “Carb Sizing” section below—preferably before you throw a Dominator on your stock Vette.

Don’t think that an out-of-the-box carb running rich is a problem. This is not a design flaw that requires instant jet changes, it’s a wise move from the aftermarket that keeps your engine from running lean.

Don’t assume that your new carb isn’t tuned properly. If you’re not building a dedicated race engine and you’re installing a brand-new, properly sized carburetor, just leave it alone. Aftermarket companies like Edelbrock and Holley put a lot of engineering into these suckers, and most of the time they’re dead-on right out of the box. It’s when you start changing things that the tune suffers.

And if you must start to change things, always increase jet sizes first. Decreasing jet sizes is a good way to lean out and possibly damage that pricey engine of yours. So if you’re gonna fiddle with the jets, go up one size first—once you know it’s running rich and losing power, then you can drop down two jet sizes.

How To Size and Buy A Carb

Choosing a carburetor is a fun yet confusing parts purchase. Here’s how to do it right.

Step 1: Use a general carb sizing formula to get into the size ballpark.

This formula looks like so:

(engine cubic inches) x (max engine rpm) / 3,456 carb cfm.

So if you have a 350 that sees a 6,000-rpm redline, simply input those numbers and calculate:

(350) x (6,000) = 2,100,000 / 3,456 = 607 cfm

Step 2: Adjust the above flow rating based on the specific engine that the carb will go on:

If your engine is stock, go one cfm rating lower.

If your engine has a larger cam or high-flowing heads, go one cfm rating higher.

Step 3: Choose your options.

If it will be going onto a mild driver, consider electric choke and vacuum secondaries. If it’s destined for the track, look into a double-pumper with mechanical secondaries and larger float bowls. Consider the type of ignition advance needed, and the type of intake manifold design that you plan on using as this affects the flange type—4150-style square-bore or spread-bore Quadrajet—as well as height and vacuum line routing/obstructions. Finally, consider details like coated vs. polished, etc.

Step 4: Pull the trigger.

If you’ve done your homework, your new street carb should reward you with a good idle and smooth transitional and WOT performance. And your new track carb should snap your neck back when the secondaries open! But of course, you can make tweaks as needed–see below.

Step 5: Tune it up.

If you’re up for spending the coin and going to an experienced dyno tuner, by all means do it. However, another, possibly cheaper way to get your tune on is to pick up a wideband air/fuel meter, like this one from FAST. It only takes a welded-in bung and wiring routing to install, and it’ll give you visual and logged air/fuel info about every move your engine makes.

After the A/F meter install, the first thing you should do is check that the fuel pressure and base/advance timing is correct. Next, take a drive with the meter recording, and be sure to idle, transition from low to high rpm through the gears and back down again, cruise, and make a wide-open throttle hit.

Then you can park it and start making changes. Start in the idle to 3000-rpm range—14.4-13.5:1 is okay for idle. Typically, a naturally aspirated engine cruising between 2300-3000 rpm likes a 14.3-13.9 air/fuel ratio, so first change your primary jets/rods, then set the idle mixtures, to hit that target. If the off-idle to 1800-rpm band is still off, non-Holleys may need the idle channel restriction enlarged to fix it. Next, go for a drive to ensure that your idle and off-idle performance is smooth, and your cruising air/fuel ratio is good. Finally, aim for 12.3-12.9 to 1 at wide-open throttle—depending on your carb type, it may require a combo of jets/rods, power valve, and accelerator pump changes to zero this in. And engines are all different, so you’ll need to track mph at the track or get dyno time to find your max power potential.

If you’d rather take a low-tech approach, you can tune idle/off-idle/cruising by sound and feel, then check your carb’s WOT vacuum readings with a gauge to determine your carb’s peak flow potential. Granted, unless you can borrow carbs from a buddy, you’ll have to do this post-purchase. But it’s still a good procedure that’ll teach you a lot about your new squirter. So, starting with the smallest carb option from the above formula, get a vacuum gauge and connect it to your engine’s intake manifold.

Seeing 1.5 inches of vacuum at WOT means you’ve nailed your carb sizing perfectly.

Seeing around 2.5 inches of vacuum shows restriction—and the need for a slightly larger carburetor.

Seeing 1 inch of vacuum or lower means the carb is too big for your engine, and it needs to be smaller.

And as always, you can check your spark plugs to see how a certain carb is running. Golden brown is perfect, while white (lean) and black (rich) means a change is needed.