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.
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.
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?
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.
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.
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.
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.