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Servicing The Rochester Quadrajet Carburetor

Renewing A Spreadbore's Performance

Moses Ludel Mar 22, 2010
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From the postwar era to the mid-'80s, General Motors' Rochester Division defined carburetor performance. Rochester also developed the mechanical fuel injection systems found on '57-'59 full-size Chevys, the '57-'58 Pontiac Bonnevilles, and the '57-'65 Corvette small-block V-8s. At the dawn of the muscle car era, Chevrolet, Pontiac, Oldsmobile, and Cadillac turned to Rochester for reliable triple two-barrel performance, and the mid-'50s Cadillac Eldorado engines featured dual Rochester 4GC four-barrel carburetors. A single 4GC carburetor was common on high-compression GM V-8s until the mid-'60s, when the introduction of the Rochester Quadrajet took four-barrel performance to the next level.

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For a variety of reasons, Corvette outsourced Carter carburetors in the early years. V-8s through 1965 used the Carter WCFB and AFB carburetors extensively. Without question, the dual WCFB four-barrel carburetion and large-bore AFB single four-barrel units established the Corvette's performance image. Despite impressive horsepower gains, potent Rochester fuel injection appealed to small numbers of Corvette buyers.

The Carter WCFB and AFB designs fit easily beneath the low profile C1 and C2 hoods. Shorter and smaller overall than the 4GC Rochester, the WCFB was the right choice for dual-quads on a compact, small-block intake manifold. Corvette would not use Rochester carburetion until 1966, when the Quadrajet four-barrel emerged.

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High performance Chevrolet muscle era engines use Holley square flange 4150-series four-barrel carburetors. Three Holley two-barrel carburetors fuel the Corvette's 427 tri-power V-8s. The L88 427, LS6, and LS7 454, the LT1 small-block and other high output small- and big-block Chevrolet engines run a single 4150 Holley four-barrel carburetor. The Holley choice reflects trackside tuning advantages rather than limitations in the spreadbore Quadrajet carburetor. An optimal fuel source for the 327 and 350 small-blocks, the Quadrajet ultimately proved itself on all lower horsepower big-block 396, 427, and 454 V-8 applications.

After 1972, Corvette dropped Holley carburetor designs altogether, finding the Quadrajet better suited for emission control use. Quadrajet carburetors, produced by Rochester and Carter (under licensed contract), had become a standard for GM four-barrel applications. Through 1981, Corvette would rely on the Quadrajet, including the electronic feedback versions. In 1982, 'Cross-Fire' throttle body injection ended Corvette's carburetor era.

The Quadrajet became the most popular and widely used four-barrel carburetor in General Motors' history. A unique "spreadbore" design, the Quadrajet embodied the best advancements in carburetion. Using precise vacuum and airflow signals, a combination of mechanical and vacuum actuated secondaries and refined fuel metering, the versatile Quadrajet accommodated engines ranging from the Buick 231 V-6 to 454 and 455 high-performance V-8s! Featuring small primary bores and large secondaries, removable jets, sized metering rod tapers, calibrated balance springs and a vacuum actuated power piston, the Quadrajet has served a wide range of engine sizes and fuel flow demands.

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Restoring The Quadrajet
'68 300 horsepower 327 and 390 horsepower 427 engines introduced Corvette owners to the '4MV' model Quadrajet. This carburetor features a divorced choke, nestled in the intake manifold at the heat crossover passage between cylinder heads. From a rebuilding standpoint, the 4MV is a prototype for other Quadrajet designs, including the attached-choke 4MC, M4MC, and the late emissions "feedback" E4ME/C models. '81-up GM vehicles with Computer Command Control (CCC) and the E4ME/C carburetors feature a more complex, electronically fuel-metered version of the Quadrajet.

Rebuilding a carburetor was once considered a routine part of tuning. If an ignition tune-up fails to remedy symptoms like hard starting, poor warm-up operation, stumble on acceleration, stalling, rough idle, excess fuel consumption, flooding, surging, or poor high-speed performance, the Quadrajet likely needs rebuilding. Before removing the carburetor, rule out ignition system weaknesses, a bad or sticking EGR valve, a sticky PCV valve, vacuum leaks, or a vacuum circuit problem. Use a vacuum pump to check the choke's vacuum break. If the vacuum diaphragm leaks, replace the vacuum break diaphragm unit during the rebuild.

Restoring the original Quadrajet carburetor will preserve your Corvette's value. You should avoid purchasing a "rebuilt-exchange" carburetor. Due to the mass-produced, similar architecture of the Quadrajet, a host of applications will fit your Corvette engine's manifold. However, neither a Buick 231 V-6 nor a 454 V-8 application will have the fuel metering components or tuning calibrations for a Corvette 350 V-8. Keep the original castings, jets, and metering rods!

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Use quality parts when rebuilding a Quadrajet. Modern carburetor kits contain extra gaskets to allow wider model coverage. Be careful when matching gaskets to the originals. Small differences can create air or fuel leaks and poor performance. The stamping number on the carburetor identifies the carburetor's original application and the gasket kit needed.

Find a clean, dust-free place to work on your carburetor. Follow the steps shown and use a factory workshop manual to confirm specifications. Compare details with the instruction sheet found in a quality rebuild kit. Properly restored, your Corvette's Quadrajet will provide years of impressive performance-and fuel savings, too!



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