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Servicing The Rochester Quadrajet CarburetorRenewing A Spreadbore's Performance From the April, 2010 issue of Corvette Fever By Moses Ludel Photography by Moses Ludel
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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. 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. Grime-encrusted, this 4MV... Grime-encrusted, this 4MV Quadrajet is typical for a higher mileage engine. Overhauling the carburetor was once just part of routine tuning. Better carburetor designs, modern gaskets, unleaded fuel, and improved sealing have extended the service intervals. Every 40,000 miles would be a good time to rebuild your Corvette's Quadrajet. 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. 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. Identification numbers, stamped... Identification numbers, stamped into the main body, tell precisely which engines the carburetor fits. This 7041211 carburetor is virtually identical to the '71 Corvette's 7041212 carburetor found on 270 horsepower 350 V-8s. One number apart, the actual origin of this carburetor is a '71 C-10 Chevrolet pickup equipped with 350 V-8. 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. 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. The vacuum break assembly... The vacuum break assembly is a crucial part of the choke and secondary air valve operation. A vacuum diaphragm pulls the choke valve slightly open at startup, enabling a rich mixture for cranking and firing, followed by a slightly leaner mixture for warm-up. Clean this assembly in alcohol to prevent damaging the diaphragm! 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! 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!
| Difficulty Index - 2 Wrenches |
| Anyone’s Project: no tools required | 1 Wrench |
| Beginner: basic tools | 2 Wrenches |
| Experienced: special tools | 3 Wrenches |
| Accomplished: special tools and outside help | 4 Wrenches |
| Professionals Only: send this work out | 5 Wrenches |  After removing the vacuum...  After removing the vacuum break assembly, remove the clip that retains the choke link rod at the choke valve's arm. Slide the rod and lever from the main carburetor body and air horn slot. Note the orientation of the rod and lever for reassembly. The rod and lever can be challenging parts to install.  The spring roll pin can remain...  The spring roll pin can remain in the top section (air horn) of the carburetor during service work. Using a tiny pin punch, gently move the spring roll pin inward to release the accelerator pump lever. If the pin slides too easily or shows wear on its friction surface, most carburetor kits contain a replacement pin. Remove the pump rod link.  Secondary metering rods are...  Secondary metering rods are calibrated and precisely shaped. Use care removing the rod holder and rods. The screw is often a Torx head. Protect these metering rods and reinstall them with care. Considered an emission control item, these rods are nearly impossible to find. Federal emission regulations prohibit the sale of aftermarket rods.  The air horn can now be removed....  The air horn can now be removed. Loosen all hold-down screws (nine in this application). Air horn lifted, you can see the accelerator pump, float assembly, power piston, and primary metering rods. Never force pieces apart. Do not pry at the gasket surfaces. The body and sealing edges are softer metal and can be easily damaged.  Gently pull upward on the...  Gently pull upward on the power piston. The sleeve seal is hard plastic and seats snugly into the main body bore. Carburetor kits do not include a new sleeve seal, which is regarded as an integral part of the power piston. Primary metering rods are brass and must be handled with care. The rod retainer spring keeps rods aligned.  Stale fuel, a dry fuel bowl,...  Stale fuel, a dry fuel bowl, or corrosive conditions contribute to carburetor troubles. Original, closed-cellular plastic floats were susceptible to damage and porosity. Surface scratches or a fingernail dent can cause a plastic float to fail. Never soak a plastic float in carburetor cleaner. A practical upgrade is the brass replacement float shown, which this carburetor acquired in an earlier rebuild.  Loosening two screws removes...  Loosening two screws removes the lower throttle body. Shown here is the gasket that seals between the throttle body and main body. Note the factory use of epoxy at the fuel chamber plugs. This added protection helps prevent seepage from the plugs. Do not remove this epoxy coating.  Verify the condition of critical...  Verify the condition of critical parts. Dip metal parts in carburetor cleaner. When selecting a cleaner, read labels and follow cleaning instructions. A California-approved cleaner will generally work as well as the highly toxic 49-State chemicals. Do not soak the vacuum break diaphragm, plastic float, or plastic float bowl insert.  Carburetor dip, a thorough...  Carburetor dip, a thorough rinse, and compressed air cleaning of all tubes, nozzles, and venturi passageways have this throttle body, main body, and air horn looking like new! When handling a carburetor or carburetor cleaner, use chemical-proof gloves, eye protection, and a respirator mask. Exposure to gasoline and VOC cleaners is unhealthy.  This thick gasket is one of...  This thick gasket is one of two in the carburetor kit. Carefully match the gasket to the throttle body and main body. Critical areas must seal properly. A leak at the gasket can spill fuel or create a vacuum loss. Take time comparing gaskets and matching parts. A foam insert, provided in the kit, helps prevent fuel seepage at fuel chamber plugs.  Each passageway must be clear...  Each passageway must be clear of debris. Brass primary jets are securely in place. Here, the accelerator pump's discharge check ball drops into the passageway. The discharge ball retainer screws into the bore. When installing jets and the retainer, use care not to damage these parts. Use a screwdriver blade that fills the slot. Tighten securely.  Install the new needle seat...  Install the new needle seat and gasket. This is brass, a soft, easily distorted metal. Use the correct driver tool or a large blade screwdriver to avoid damaging the seat. If the slightest amount of brass sloughs off during tightening, vacuum or air blow debris from the area and passageways. Tighten securely but do not distort the seat.  The float hinge pin and needle...  The float hinge pin and needle have been installed here. The needle has a tiny spring retainer that must be in place. Note the use of a brass float, always a practical upgrade from the factory's plastic or phenolic resin type. A plastic float bowl insert secures the hinge pin and prevents fuel slosh in the float area.  With the hinge pin seated,...  With the hinge pin seated, gently tip the back of the float arm and lightly press the needle against its seat. Do not use excessive force, or you will damage the new needle's tip. Hold the float up and measure the float height in this manner. Float height is critical, determining the operating fuel level in the bowl.  This is the power piston with...  This is the power piston with primary metering rods in place. Note the position of the retaining spring. This spring is important, keeping the rods in place and tensioned. The piston retainer sleeve is part of the assembly and reused. Protect this sleeve. Install it firmly in the bore after aligning the metering rods with the jets.  Primary metering rods and...  Primary metering rods and the float retainer/bowl insert are now in place. Float arms must move freely, clear of the carburetor body. Rods must also move freely without interfering with other parts. Primary rods have machined tapers, factory matched to the jet sizes for your engine-another reason to keep the stock carburetor.  A new accelerator pump assembly...  A new accelerator pump assembly may or may not come with springs. If not, remove and install the original springs onto the new accelerator pump. Coat the accelerator pump bore with a light film of motor oil. This will prevent friction damage to the pump seal prior to fuel entering the bowl.  Polished metering rods for...  Polished metering rods for secondaries are ready for installation. The air horn is in place and retained with nine screws in this 4MV application. Recheck the screw torque as the gasket settles. Slide metering rods carefully into their bores. Use Loctite 242 on the rod hanger's tiny screw threads. Tighten the screw securely to prevent loosening in service.  Drop the choke link and lever...  Drop the choke link and lever carefully into the cavity. This is easier with the carburetor free to turn on its side. Aligning the lever and keeping it in place during vacuum break installation is tricky. If the lever falls off, fish it out gently with a piece of mechanic's wire. With patience, you will get these parts aligned.  Carburetor tilted on its side,...  Carburetor tilted on its side, a screwdriver indexes the lever, aligning it with the intermediate choke shaft. Keep the rod on the lever and the lever aligned. With the rod and lever aligned, make sure the rod fits into the choke valve arm. Secure the rod with a new horseshoe clip. Make sure the clip will not come loose in service.  Make sure the link rod is...  Make sure the link rod is in place as shown. You must hook the link before bringing the vacuum break into position. Install the fast-idle cam and secondary lockout lever before aligning the intermediate shaft with the intermediate choke lever's slot. Move the vacuum break into position. Use Loctite 242 on the retaining screw's threads.  Accelerator pump arm fits...  Accelerator pump arm fits as shown. This application uses the inner hole for a shorter stroke. Note the linkage position. The spring roll pin fits securely, centered to provide support for the arm. Make sure the arm's pivot hole is not worn badly. Fortunately, these arms can be matched readily if a replacement is necessary.  Polish needle ends of idle...  Polish needle ends of idle mixture screws. Install springs and turn screws to the bottoming point-gently. Do not seat the screw tightly. Back each screw out the number of turns listed in the factory specifications. Adjust these screws after engine warm-up, following service and tuning guidelines. Later applications have limiter caps or sealed screws.  Install a new vacuum break...  Install a new vacuum break hose-avoid kinks. Earlier Quadrajet models are prone to inlet thread failure. Damaged fuel inlet threads should be restored properly with a Heli-Coil thread insert. (Don't waste time with other methods.) A pleated paper filter fits behind the fuel inlet nut. Install gasket and nut. Tighten nut to 25 ft-lbs.  Vacuum break link clearance...  Vacuum break link clearance is one of the critical measurements. In the instruction sheet, you will find references to each important adjustment. Follow details when adjusting linkage rod lengths and the secondary lockout. The choke and secondaries will not function properly unless these adjustments are correct.  The choke opens slightly as...  The choke opens slightly as the vacuum break applies. This is an important measurement, listed on the specification sheet. As the engine fires, vacuum applies at the diaphragm, pulling the arm and choke lever. Use a quality gauge to measure the adjustments. Follow the adjusting sequences carefully.  Secondary linkage adjustments...  Secondary linkage adjustments are crucial to Quadrajet performance. These adjustments determine whether the engine will respond properly under partial and full-throttle acceleration. For the 4MV models, there are "3-point pickup" and "2-point pickup" linkage systems noted in factory workshop manuals. Identify your carburetor's type.  The choke opens slightly as...  The choke opens slightly as the vacuum break applies. This is an important measurement, listed on the specification sheet. As the engine fires, vacuum applies at the diaphragm, pulling the arm and choke lever. Use a quality gauge to measure the adjustments. Follow the adjusting sequences carefully.  Secondary air valve's spring...  Secondary air valve's spring tension is precisely calibrated. If the tension needs adjusting or you suspect that it is incorrect, follow adjustment steps in your Corvette factory workshop manual. Match a new base gasket to carburetor and manifold. Torque the carburetor mounting bolts and nuts to 14 ft-lbs. Do not over-tighten. Recheck the mounting hardware torque after engine warm-up. Job done!
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