EngineLabs’ Carb Science Series: Inside Edelbrock’s AVS2 Carburetors

The cliché contends you can’t judge a book by its cover, and that might just fit the concept of Edelbrock’s new AVS2 street carburetor. A quick glance at the exterior of the AVS2 doesn’t reveal why this carburetor has become extremely popular with the street set. You have to look a little deeper.

The secret really isn’t anything new — but it does work. Often it’s the little things that make a carburetor work better. A couple of years ago, Edelbrock’s recently-retired Manager of Testing, Curt Hooker, suggested making a change to the classic Edelbrock AVS (or Adjustable Valve Secondary) line of carburetors.

Hooker suggested adding what is called an annular-discharge booster to the primary side of the carburetor to improve its part-throttle performance. This simple upgrade made a significant difference in throttle response. Edelbrock sent one out for us to try on a mild 383small-block Chevy with a TH350 transmission.

While our impressions of this carb are purely subjective, we can tell you that the difference in initial throttle response is immediately noticeable compared to the previous version. Rather than just drop that seat-of-the-pants report at your doorstep, this is the Carb Science Series, so let’s get into why this annular booster is so much better than its predecessor.

Edelbrock makes it easy to identify the AVS2 carburetor with its large tag on the front of the carb body. Otherwise, it looks the same, externally, as the previous AVS. The AVS2 comes in 500, 650, and 850cfm sizes.

Discharging Performance

Most street carburetors employ what is called a single-outlet booster. The booster’s function is to deliver main metering circuit fuel into the venturi, where the incoming air shears the fuel, hopefully turning it into very fine droplets. Unfortunately, quite a bit of the fuel leaving the booster (especially at low air velocity and engine speeds) remains in the form of large globs. Large droplets of liquid fuel must be broken down into smaller droplets before they will efficiently contribute to combustion.

An annular booster uses a more efficient design. When the fuel from the carburetor main-well enters the booster, it circulates around a small groove located around the circumference of the booster. Engineers call this groove an annulus. Located on the inside diameter of the booster are multiple small holes connecting to the annulus. Fuel discharges from each of these tiny holes, which shears the fuel much more efficiently. Smaller droplets of fuel burn more easily and quickly than large ones.

(Left) Early production carbs used a natural finish booster while later versions now use a red outline around the annular boosters. The annual boosters, which are only employed on the primary side, are the key to the AVS2’s excellent throttle response. (Right) This line of carburetors gets its name from a spring-loaded door over the secondary venturi. As inlet-air velocity through the primaries increases, this puts pressure on the secondary air valve. When sufficient load overcomes the spring pressure (which isn’t very much), the door opens.

Not only does the annular booster create a finer mist of fuel than a standard booster, but it also initiates the flow of fuel sooner, because of its multiple discharge holes. The net result is far more response off-idle and, at least in theory, the potential to improve fuel mileage.

Some may question that statement since it sounds like more fuel is delivered compared to a single-hole discharge booster. This is true, except that Edelbrock has completely re-calibrated the AVS2 to compensate for this more responsive booster, effectively delivering a similar amount of fuel for the same engine demand.

What this does is, leave the door open for a sharp tuner to further calibrate the carburetor’s part-throttle operation based on the demands of the individual engine in question. The potential is there to gain even greater throttle response along with a slight increase in mileage. Who wouldn’t vote for that?

Here’s a shot of the original booster design used in previous AVS Edelbrock carbs. Also, note that all the attaching screws use a Torx head. To disassemble this carburetor, you’ll need a T-25 Torx screwdriver.

Response Time

This makes the AVS2 a highly responsive carburetor for mild street applications. The AVS2 is offered in 15 different configurations including 500, 650, and 850cfm sizes, and in both manual and electric choke versions. There’s also an Endurashine option that employs a high-tech coating to maintain its factory-like finish.

Beyond the size options, one reason the Edelbrock AVS2 line of carburetors is so popular is they offer some very simple opportunities to improve performance and drivability. The primary metering circuit is designed similarly to the GM Rochester Quadrajet by employing the combination of a main jet with a metering rod. We’ve included an Edelbrock drawing that illustrates this very well.

(Left) This is an Edelbrock illustration of how fuel is delivered in an older, single outlet booster Edelbrock carburetor. Especially at lower engine speeds, the fuel tends to exit the booster in large clumps or droplets of fuel that do not easily vaporize. (Right) Note how the annular booster in this illustration introduces the fuel through multiple small holes around the inside of the annular booster. These smaller holes do a much better job creating small droplets of fuel to easily vaporize and burn more efficiently in the combustion chamber. This is what enhances the low-speed throttle response.

The primary metering system is designed with a tapered metering rod attached to a power piston, which is supported by a calibrated spring. Under light-load conditions (which is roughly 80- to 90-percent of carburetor operation), engine vacuum compresses the spring and places the larger portion of the tapered metering rod in the jet. This reduces the jet’s flow area, allowing the engine to run at a leaner air-fuel ratio.

When the throttle is opened to increase power, engine vacuum will drop, and the spring pushes the piston and metering rod upward. This positions the smaller, tapered portion of the metering rod in the jet, which will flow more fuel and create a slightly richer air-fuel ratio for more power.

What makes the Edelbrock AVS2 line of carburetors so handy is that this metering rod, piston, and spring assembly is easily accessed with a simple loosening of a screw and removal of a small cover plate. There’s no need to remove the entire lid from the carburetor unless you choose to change the primary or secondary jets.

This illustration shows how the metering rod and jet combination operate at both low-load, high-manifold-vacuum situations where a leaner air-fuel ratio is used (left). Conversely, in the high-load, WOT situation (right) the metering rod is pushed up by the power-valve spring, allowing for a richer mixture.

Secondary Benefits

On the secondary side of things, this line of AVS and AVS2 carburetors gets its name from the spring-loaded door over the secondary venturi. Carter called this an Air-Valve Secondary (a slightly different “AVS” definition than Edelbrock). These carburetors are all actuated with mechanical secondary linkage yet they do not use a secondary accelerator pump circuit.

If all four barrels are opened at low speed (such as from a dead idle), the engine does not need airflow through all four barrels to make power. Once engine RPM begins to climb and inlet-air speed increases, inlet-air velocity overcomes the light spring pressure holding the secondary air-valve door closed. Once the door begins to move, it usually opens quickly, allowing air velocity at sufficient speed to initiate fuel flow from the boosters. This eliminates the need for a secondary accelerator-pump circuit.

Part-throttle tuning of both the AVS and the AVS2 is very easy. All you have to do is loosen that little screw and slide the cover out of the way. The primary metering rod, piston, and spring package are now easily accessed. The entire lid of the carburetor is only removed if you choose to make changes to the primary or secondary jets.

Tuning the secondary air-valve door is really easy, yet it is seldom required. Edelbrock pre-sets the tension on the air-valve-door spring, and for most applications, it does not need adjusting. Reducing the spring tension allows the door to open sooner. But, if the tension is too loose, it will cause a hesitation, which is the result of slow insufficient airspeed to begin secondary booster fuel flow. Some enthusiasts contend that they can “feel” the secondary open. That’s great — as long as what they “feel” isn’t a hesitation.

All these features make the Edelbrock AVS2 an excellent carburetor for most mild street applications of less than 500 horsepower. But, its real claim to fame is fantastic throttle response right out of the box. Ironically, in the 21st century, we’re still tweaking carburetors. But the truth is, for a typical small- or mild big-block street engine, it doesn’t get much simpler than an Edelbrock AVS2.

Edelbrock offers a separate calibration package with jets, metering rods, and power valve springs that allow a wide range of air-fuel possibilities for part- and WOT tuning.

Article Sources

About the author

Jeff Smith

Jeff Smith, a 35-year veteran of automotive journalism, comes to Power Automedia after serving as the senior technical editor at Car Craft magazine. An Iowa native, Smith served a variety of roles at Car Craft before moving to the senior editor role at Hot Rod and Chevy High Performance, and ultimately returning to Car Craft. An accomplished engine builder and technical expert, he will focus on the tech-heavy content that is the foundation of EngineLabs.
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