Rochester Quadrajet carburetors from Hemmings Muscle Machines

January, 2005 - Jim O'Clair

Rochester isn't exactly the first name that comes to mind when you are thinking of a performance carburetor. In past months, we have discussed Holleys and Carters that can be scavenged from the relics in your local pick-a-part to provide good bolt-on performance at a cheap price. Although the Quadrajet carburetor is not as popular as the others, it is still a very good alternative for many spread-bore 4-barrel applications when trying to find a little more economy and performance for your hard-earned dollar.

First released by Rochester in 1965 and used extensively in GM cars from late 1967 on, The Quadrajet was the evolution of the earlier Rochester 4GC carburetor; however, it also incorporated ideas from the other manufacturers, such as the spread-bore design and vacuum secondaries. They are stingy on gas at lower speeds, which makes them more fuel efficient than their competition, but their vacuum secondary circuit can give your car the added performance boost it needs, once they are engaged. For example, a Quadrajet, in primary circuit (at idle between 1-1/2" and 3" Hg) only operates at 160 to 225 CFM, whereas a Holley 4160 idles at between 300 and 425 CFM. Q-jets are also highly adaptable to different engine sizes with the changing of primary and secondary metering rods and jets. Earlier 4GC models were only available in 486, 553, or 692 CFM. Quadrajets can range anywhere in between 600 and 800 CFM. Most are rated 750 CFM or higher.

Quadrajets are used by many GM street car enthusiasts; however, they are also disliked by just about as many. The Quadrajet has been getting a bad reputation with a lot of muscle car owners and online discussion boards looking for more "get up and go." This is because rebuilding them is not quite as easy as a Carter AFB or a 4160 Holley. But parts are easy to find, and a properly rebuilt unit will perform just as well as the other brand names, if not better. In fact, Chrysler used Quadrajets extensively, in the late '80s, proving their popularity over the Carter Thermoquad with the engine designers at Mopar. In addition to Rochester, Carter Carburetors was also contracted to build the Quadrajets for GM cars from 1966 into the late '70s. It is not uncommon to see a Carter-built Quadrajet with a Rochester part number on it. Carter even made some aftermarket units that had a Carter triangular identifying tag on them. The standard 4-digit Carter part number followed by an "S" would mean these particular Quadrajet carburetors were aftermarket replacements. Any and all replacement parts between Carter and Rochester are directly interchangeable.

There have been quite a few models of Quadrajets used over the years, which proves they can be adaptable to changing conditions. The first few versions to appear were referred to as the 4MC, with a side-mounted integral choke, and the 4MV, which used a manifold-mounted choke (there was also a model 4M which used a choke cable instead of the bimetal choke coil). These three early versions were, for the most part, the same units internally. As we mentioned, the Quadrajet is a spread-bore design carburetor, using 1-3/8" primary bores and 2-1/4" secondaries. The smaller primary bores give the carburetor better idling and low-load fuel economy. The larger secondary bores are regulated by tapered metering rods which set the air to fuel mixture.

Quadrajets are usually mounted above a thicker manifold gasket to protect the carburetor from engine heat and reduce fuel percolation. This also prevents the carburetor from being over-torqued onto the manifold and causing warpage and vacuum leaks, another common but unwarranted complaint about them. Q-jet experienced rebuilders can correct this warping problem too. The issue of leaking vacuum around throttle shafts can be addressed with the installation of new throttle shaft bushings as well.

All Rochester carburetors, including the Quadrajet, are identified by a 7-digit or 8-digit part number; the earliest units, made before mid-1968, had a round metal tag attached to the fuel bowl. The 7-digit part number ran around the outside edge of the tag. In late 1968, the 7-digit part number was stamped vertically into the main housing, just behind the secondary throttle plate linkage. The part number of the carburetor you are able to locate will tell you some information about when it was built and where it came from originally. This allows you to know what type of modifications might be necessary to adapt it to your particular engine size and horsepower.

Rochester carburetors with a 7-digit part number starting with 702 through 704 were built before 1976. On these pre-1976 units, the next number immediately after these three digits, will be a "2," if the carburetor is indeed a Quadrajet. Occasionally you will see some with a "5" but these are for California emission standards carburetors. The next digit following the "2" will indicate the type of GM car it was taken from originally. A "4" would mean it came from a Buick, a "5" would indicate Oldsmobile. Pontiacs are usually a "6" or a "7," and Chevrolets would have its next digit as a "0," "1" or "2." The last two digits can indicate whether the carburetor came from an automatic or manual transmission-driven car. Usually an even number will indicate an automatic; odd numbers indicate a manual transmission. Cadillacs, however, do not follow this rule of thumb. They could be numbered with any of the above model designation numbers. The post-1976 units do not follow the above rules. Eight-digit casting numbers began in 1976. These carburetor identification numbers start with 1705 or 1708, depending on whether they were built in the '70s or '80s; 1705 series numbers would identify 1976 to 1979 carburetors. For the 1980 and up carburetors, the fourth and fifth digits are the last two digits in the year in which it was produced. The last three digits give you no information about the carburetor without a Rochester numerical reference chart. Searching for one of the three early-style Quadrajets should be relatively simple. You can look at these cars and trucks to find one:
o 1965-66 Chevy w/327, 396 or 427.
o 1967-74 Chevy w/350, 400 or 454.
o 1975 Chevy w/454.
o 1970-71 Avanti.
o 1967-74 Buick 350, 400, 430 or 455.
o 1968-74 Chevy truck V-8 light-duty.
o 1975-78 Chevy truck heavy-duty.
o 1967-72 Pontiac w/350, 400, 428, 455.
o 1967-74 Olds w/330, 400, 425 or 455.

Quadrajets built before 1972 were 750 on the 4M, 4MC and the 4MV. After 1972, the CFM rating on all part numbers for these three models was raised to 800.

There are later versions of the Quadrajet that were also produced as additional equipment such as onboard computers, mixture control solenoids and throttle position sensors were added. Model M4MC was used in GM 4-barrel applications from 1975-79. It used the standard carburetor-mounted choke and dual choke pull-offs. The model M4ME was added in 1976 and is similar to the M4MC but used an electric choke. Both of these models can make acceptable substitutes for the two earlier 4MC and 4MV applications. The M4ME is the model that was used on 1985 to 1988 Dodge trucks with a 360 engine as well.

Engine management computers brought about the next change in 1981, with the introduction of the E4ME. These units had an electronic mixture control solenoid added to them and used an electric choke. Nineteen eighty-six to 1990 Buick and Olds used a model E4MC, which is the same unit as the E4ME, but the fuel inlet faces directly into the front of the carburetor, instead of the inlet connecting to the front from a 90-degree angle. Fuel line clearance can be a problem for the E4MC carburetors because of the water outlet location on your car. Both of these later models also use a throttle position sensor to supply information to the onboard computer. The computerized carburetor models were rated only at 600 CFM, (even though they are capable of running at 750) because the travel of the secondary air valves has been limited. A competent rebuilder who knows his way around a Q-jet can modify these units to give you the higher CFM rating. Depending on the emissions equipment you are using in your car, and whether it is computerized or not, the E4ME and the E4MC are probably not models of the Quadrajet you should be considering for an upgrade. These models do work well, though, in their original applications, such as third-generation F-Body Camaros and Firebirds or late-model G-Bodies like the Monte Carlo or Regal. But using them in an earlier-generation car would not give you any added benefit.

Changing jets and metering rods in a Quadrajet is not quite as easy as an AFB, but the principles are the same. Jets are numbered to indicate their orifice size (and their replacement part number too). Primary and secondary metering rods are also marked for easy identification. Primary rods are stamped with a 2-digit number indicating their diameter, between .033 and .050. Secondary rods are marked with a two-letter designation which references to specific diameters at three points along the secondary metering rod. Diameters are measured at low idle, under load and wide open throttle positions, Secondary metering rod hangers can also be changed to raise or lower the secondary metering rod positioning within the jets.

Quadrajets can be used in any standard-shift Ford, Chrysler or GM spread-bore 4-barrel manifold applications. They can be substituted for any Carter Thermoquad, Holley 4165 or 4175, and Motorcraft 4100 and 4300 spread-bore models with few modifications (sorry AMC builders, the Motorcraft 4350 is not compatible with any of these other spread-bores). Q-jets also work well in cars with automatic transmissions that use a cable-operated kickdown for shifting, but can also be adapted to a linkage style kickdown setup. So, basically, they can be retrofitted onto Fords later than 1964, GM cars after 1965 or Chrysler products after 1971 (as well as International Scout after 1973) where a spread-bore manifold was used.

They can be also be fitted to square-bore manifold applications that originally used the Carter AFB, Rochester 4GC, or Holley 4160 and 4150 series carburetors. Quadrajets would require an adapter plate to convert both the bolt pattern and fuel distribution pattern to fit the spread-bore manifold. The adapter compensates for the oversize secondaries being offset towards the rear of the intake opening. Finding a compatible donor carburetor for your application should be relatively simple. There are hundreds of thousands still on today's highways or sitting in your local salvage yard. Used units can go as cheap as $50.00 to $150.00, and various auto parts stores have rebuilts available for between $150.00 and $275.00, depending on the Rochester model you decide on. At that kind of pricing, they are certainly a far cry from the new aftermarket Quadrajet, Holley, Carter or Demon carburetors. And they can also be a low-budget performance alternative in terms of price in the short term and in terms of gas mileage in the long term. Their adaptability allows you to try multiple jet and metering rod setups to find the one that works best for you for your daily drive and what other configuration will work best for the weekend at the strip too. The keys to making this swap work are in the footwork (finding a suitable replacement) and in the execution (tuning the carburetor to fit your specific needs). I would not recommend that you attempt to rebuild a Quadrajet without a repair manual handy.

Rochester Carburetors by Doug Roe and Haynes Rochester Carburetor Manual by Mike Stubblefield and John Haynes can both be a great technical resource for the rebuilding process. Or, if you can find a good Rochester rebuilder who knows how to modify it to where you want it, that would probably be the best advice I can give you. Jet Performance and/or Dennis Barker are two really good rebuilders who come to mind.

More QJ Info

There are a lot of myths surrounding the E4ME version of the Q-jet. Until GM's Rochester Division could create reliable fuel injection systems, it was really a very simple solution to Federal Emissions mandates. The E4ME did its job so well that Ford, Chrysler, and AMC all bought them from GM to use on some of their higher output engines of the time.
The Quadrajet was simply redesigned to accept electronic internal components, offering more precise metering of the primaries, reducing emissions, and improving fuel economy at part open throttle. The CCC's Electronic Control Module (ECM) monitors several engine parameters under normal operating conditions and utilizes an oxygen sensor in the driver's side exhaust manifold to measure those adjustments. These carbs use a Mixture Control Solenoid (MCS) & a Throttle Position Sensor (TPS). The TPS sends electronic signals to the ECM, then based on the various sensor info, the ECM sends varying signals to the MCS within the carburetor, controlling the depth of the primary metering rods in their jets, richening or leaning the mixture based on engine load.

In Closed Loop, the computer moves the MCS up & down to achieve the ideal 14.7 Air/Fuel Ratio. The TPS sends signals to the computer and tells it where the throttle is positioned. The Rich and Lean Stops limit the travel of the main Metering Rods. The Idle Air Control Valve (IAC) adjusts the Idle Air bypass. The Oxygen Sensor (O2 Sensor) reads the exhaust gases for the Rich/Lean condition of the exhaust gases and sends those readings in pulses to the ECM, O2 Lean = wide pulse, O2 Rich = narrow pulse. Again, the computer only controls the carb's primary fuel metering circuit at idle and part throttle, so once you hammer the pedal and open the secondaries, its all yours.

One common thing that most people don't realize is that the computer (ECM) also controls the Distributor's timing advance/retard. If the computer isn't reading all of the correct parameters, it will pull timing and run like crap, aka goes into "Limp Home Mode". Another very important thing to note is that the ECM has a block learning capability, ie; it saves mixture values to adjust for driver habits, normal wear, and seasonal weather conditions. After every tune up, or anytime you replace anything on the engine, it is very important to unplug the battery for at least 15 minutes to reset the computer's memory back to the original factory baseline so the CCC system can readjust to the new tune up &/or parts.