In its stock form, the Rochester Quadrajet carburetor often gets a bad rap, but with a bit of work, it can be a very capable performer.
The long-discontinued Rochester Quadrajet carburetor remains popular with performance rebuilders and tuners.
Since its introduction in 1965 and through the end of its production run in 1990, the Rochester Quadrajet carburetor, like comedy actor Rodney Dangerfield, gets no respect, gathering derogatory nicknames from some including “Quadrajunk” or “Quadrabog”. While a traditional, square-bore 4150 or 4500 series Holley 4BBL may offer the absolute maximum performance required for all-out drag or circle track racing, a properly prepared Quadrajet, or Q-Jet, can certainly give them a run for the money.
A spread-bore by design, the Rochester Quadrajet is particularly effective in street form, with small primaries that offer exceptional throttle response, yet good fuel economy, while its large air valve-controlled secondaries have the ability to deliver maximum, “seat-of-the-pants” performance in an instant. Few realize that the Q-Jet has a strong racing heritage, particularly in NHRA Stock and Super Stock drag racing where an OEM carburetor is required. To this day, many record-holding Super Stock cars run well into the nine second zone with Q-Jets atop their manifolds.
The late John Lingenfelter, a legendary drag racer and engine builder, was among the first to turn the Rochester Quadrajet into a record-setter on his Super Stock Chevys in the early 1970’s. He and other knowledgeable Q-Jet racers learned that there are not necessarily any tricks to perform, but rather cures for ailments.
Two basic sizes of Quadrajet were produced over the years; 750 and 800 CFM versions. The 800 CFM unit (identified by a slightly larger primary venturi) was relatively rare, found only on 1971-73 Buicks and 1973 Pontiacs. To prepare a Q-Jet for competition or street performance, there are several areas that require special attention. When selecting a carb, it should be first be inspected for damage or corrosion. Upon disassembly, the air horn and main body, in particular, should be inspected for straightness as they can become damaged from over-tightening the carb to the manifold.
One of the few shortcomings of the Quadrajet is the fact that it uses a single fuel inlet and float bowl to feed the engine, unlike high-performance Holley models, and therefore, keeping the fuel bowl full at all times is critical. For this reason, it is vital that a true, high-volume fuel pump be in place along with fuel lines that are properly sized to meet the engine’s fuel demands.
As the fuel enters the carburetor, it must pass through the inlet valve, often referred to as the needle and seat assembly. Delco-Rochester produced three different sized inlet valves: 0.110″, 0.125″, and 0.135″. The 0.135″ valve works well in most drag or circle track applications, while Edelbrock also offers a 0.145″ high-flow assembly (EDE1980) for fuel-thirsty machines. Primary jets are the screw-in replaceable type ranging from 0.066″ to 0.078″, also available from Edelbrock (EDE19– specify size). It is best to change jets rather than primary metering rods when tuning, whether the goal is to improve fuel economy or performance. Primary jet size is commonly in the 0.068″ to 0.075″ range.
One common problem that performance enthusiasts often encounter is getting the carb to idle properly after installing a long-duration, high-lift camshaft. Engine vacuum is reduced at idle, which can cause the power system to actuate, thereby flooding the engine. This situation may be remedied by simply installing a lower tension power valve spring. Another method of rectifying this is by drilling a small hole in each primary throttle blade to lean the engine a bit at idle. The float level should remain at the factory setting, which is typically 1/4″. When adjusting engine idle, it is important that the primary throttle blades remain in line with the throttle body transfer slots, or performance, fuel economy, and driveability will suffer.
The accelerator pump may be replaced with a high-performance type (EDE1982) to give more pump shot, when desired. Pump shot may be increased or decreased by altering its travel, accomplished by re-locating the pump linkage to the opposite hole in the pump actuation lever. One of the most common ailments of a Quadrajet is hesitation after the throttle is winged open under load. Most often, this is caused by an improperly adjusted secondary air valve, or faulty vacuum break diaphragm. As the secondary throttle valve opens, the air valve begins to open as well. If the air valve opens too quickly, the engine will hesitate, or “bog”.
Will not accelerate to its potential
On the other hand, if the valve doesn’t open quick enough to meet the engine’s air/fuel demands, the engine will not accelerate to its potential. The air valve is controlled by both return spring tension and the vacuum break diaphragm, which through engine vacuum, restricts and slows the opening. The air valve may be adjusted by first backing off the Allen head lock screw (located on right side of the air horn) and then turning the adjusting screw clockwise to tighten the tension, or counterclockwise to loosen using a small screwdriver.
Secondary Air Valve
Additionally, as the secondary air valve opens, the secondary metering rods are raised out of the secondary metering orifices. Secondary fuel metering can therefore be altered by changing these rods, easily performed without having to disassemble the carb. Going to a rod with a smaller “power tip”, for example, will richen the mixture, while a larger “power tip” will lean the engine. Most performance applications utilize rods with “power tips” in the 0.030″ to 0.050″ range. Edelbrock offers 0.041″ (EDE1951) and 0.066″ (EDE1954) metering rods for performance tuners.
Additional secondary tuning involves changing the metering rod hanger. Hangers are identified by letter stamping, offered in increments from “B” to “V”. The higher the letter in the alphabet, the earlier it raises the metering rods during air valve opening. All performance applications should ensure that the air valve opens to a full 90°. Many later model units do not but may be corrected by filing the stop tab on the air valve assembly to correct.
Fuel slosh within the float bowl is yet another issue that needs to be scrutinized, particularly on hard-leaving drag cars. Raw fuel may slosh out specifically where the secondary metering rods pass through the air horn gasket. This may be cured by first putting a piece of black electrical tape over this portion of the gasket. Follow by making two small “X”-shaped slits in the tape with a razor blade, which allows the metering rods to pass through this taped area. Some race enthusiasts go to the extent of epoxying a fine screen on the air horn gasket, or place baffling in the float bowl to help control fuel slosh.
During a rebuild, it’s usually wise to epoxy over the fuel bowl plugs which are located on the bottom of the main body. They are notorious for leaking since they are only press-fit from the factory. Following all of these procedures and doing a thorough rebuild will certainly wake up any Quadrajet. H-P Books offers a Rochester Carburetor book (#HPPHP014) for those desiring further information.
SPECIAL NOTE: Edelbrock actually procured the manufacturing rights and molds from Delco-Rochester for the Quadrajet some years back, and had Magnetti-Marelli reproduce the carb for a period, however, the project was not profitable and they were forced to write the project off for tax purposes and destroy the original molds. Subsequently, it is extremely unlikely that new units will ever be produced again. Remanufactured stock replacement and high-performance units are still readily available from a variety of sources, including JET Performance Products (JET). I have an interesting post on Camshaft Lift.
This article was written By Jim Kaekel, Jr. I hope you enjoyed this post on Rochester Quadrajet.