Carter's AFB carburetor, introduced...
Carter's AFB carburetor, introduced to the automotive industry in 1957, became Corvette's choice for the 300 and 340 horsepower RPO 583 and RPO 396 options in 1962. Exceptional stability and predictable performance were AFB hallmarks.
At the dawn of the muscle car era, GM had thoroughly tested Carter's AFB carburetors on Chevrolet's 348 and 409, and on the high-output Pontiac 389 V-8s. The AFB was a logical choice for Corvette's new 327 small-block V-8s. From 1962-'65, a single, high-flow Carter AFB four-barrel carburetor fueled the 300 and 340 horsepower engines. Until the 1964 introduction of the Holley 4150 carburetor on 365 horsepower L76 engines, only Rochester's mechanical fuel injection could top the AFB's performance.
Carter's AFB (aluminum four-barrel) made history from 1957 through the 1960s. For high-performance engines throughout the industry, the AFB and its successor, the AVS, were common picks. Topping the muscle car ratings for 1962, Chevrolet's big-block 409, equipped with dual AFBs, produced 409 scorching horsepower. Not outdone, the Corvette squeezed 340 horsepower from a mere 327 cubic inches, using a performance mechanical lifter camshaft, 11.25:1 compression, and a single Carter AFB carburetor!
This is a typical high-mileage...
This is a typical high-mileage AFB. Dirt, grime, clogging of jets, and gummy venturi occur over time. The carburetor that sets up for long periods with stale gasoline in its bowls is a candidate for problems.
The needs of an AFB carburetor are minimal. Unlike the finicky Holley square flange designs, the AFB holds a tune well. Less susceptible to altitude and atmospheric effects, the AFB requires less tinkering at trackside. Owners of 300 and 340 horsepower 327s attest to the driving pleasure and tuning ease that the AFB offers.
Among the desirable features of the AFB is its "dual two-barrel" design. The primary barrels functioned as a true two-barrel carburetor, featuring low-speed, high-speed, accelerating, and power systems. The choke, as customary, operates on the front two barrels. Secondary throttles open mechanically as the throttle linkage reaches a predetermined stage. Vacuum actuated auxiliary valves smooth the transition into the secondary barrels. The mechanically actuated linkage, complemented by counterbalanced and vacuum actuated auxiliary valves, assures predictable and stable performance.
The track tuning of an AFB is not difficult. Vacuum operated step-up pistons have calibrated counterbalance springs. Metering rods attach to these pistons, which can be accessed with the air cleaner removed and without removing the carburetor's air horn. Float adjustment does require removal of the air horn. Once adjusted properly, the two floats hold their settings between overhauls.
Varnish buildup accrues. Spillage...
Varnish buildup accrues. Spillage from worn gaskets creates leaks. Gaskets shrink and loosen over time, which leads to gasoline seepage. Leaded fuel leaves deposits in the bowl floors. This rebuildable carburetor looks typical for its mileage.
Due to its spare design, the AFB is reliable and comparatively easy to service. A properly tuned AFB provides easy starts, a stable idle, and quality performance-without tinkering. These carburetors, like any other alloy type, are vulnerable to throttle shaft wear and main body throttle bore wallowing. In that regard, however, the AFB holds up better than most.
One of the most satisfying aspects of AFB work is the refinement of tune. An AFB's adjustments, like other carburetors, are critical and must be within close tolerances. Following that standard, the rebuilding process focuses on the various features and adjustments required. With adequate time spent, a builder can fine-tune an AFB to perform as designed. Once set properly, the AFB provides exceptional driving satisfaction.
Check each throttle shaft...
Check each throttle shaft for side play. Remove the step-up pistons, springs, and rods. These rods meter fuel under various engine loads and throttle positions. Rods slide up and down within replaceable jets. Jets screw into the main body floor.
Rebuilding always involves careful disassembly and cleaning. Sequence of disassembly is important, as the delicate metering rods and other brass parts require protection. For years, parts like the rods have not been available outside of NOS trackside tuning kits. (These rare and obsolete NOS kits have sold for $1,000 or more!) Avoid damaging parts. A numbers-correct AFB carburetor, in rebuildable condition, is a valuable part of your engine package.
Rebuilding and tuning were once routine, periodic service measures. During the high-octane, Tetra Ethyl Leaded-fuel era, a Corvette equipped with an AFB would require carburetor rebuilding-or at least a "boil-out"-every 25,000 miles. With a quality rebuilding kit and proper adjustments, a rebuild should last at least that mileage. One deciding factor would be how much the car sets idle, as varnish buildup and jet clogging occur rapidly as fuel decays. Use of a fuel stabilizer would be wise if your Corvette sets up for long intervals.