If the air valve opens too quickly, the engine will hesitate, or “bog”. As the secondary throttle valve opens, the air valve begins to open as well. Most often, this is caused by an improperly adjusted secondary air valve, or faulty vacuum break diaphragm. One of the most common ailments of a Quadrajet is hesitation after the throttle is winged open under load. 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. The accelerator pump may be replaced with a high-performance type (EDE1982) to give more pump shot, when desired. 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 float level should remain at the factory setting, which is typically 1/4″. Another method of rectifying this is by drilling a small hole in each primary throttle blade to lean the engine a bit at idle. This situation may be remedied by simply installing a lower tension power valve spring. Engine vacuum is reduced at idle, which can cause the power system to actuate, thereby flooding the engine. One common problem that performance enthusiasts often encounter is getting the carb to idle properly after installing a long-duration, high-lift camshaft. Primary jet size is commonly in the 0.068″ to 0.075″ range. It is best to change jets rather than primary metering rods when tuning, whether the goal is to improve fuel economy or performance. Primary jets are the screw-in replaceable type ranging from 0.066″ to 0.078″, also available from Edelbrock (EDE19– specify size). 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. Delco-Rochester produced three different sized inlet valves: 0.110″, 0.125″, and 0.135″. 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.Īs the fuel enters the carburetor, it must pass through the inlet valve, often referred to as the needle and seat assembly. 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. 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. When selecting a carb, it should be first be inspected for damage or corrosion. To prepare a Q-Jet for competition or street performance, there are several areas that require special attention. The 800 CFM unit (identified by a slightly larger primary venturi) was relatively rare, found only on 1971-73 Buicks and 1973 Pontiacs. Two basic sizes of Quadrajet were produced over the years 750 and 800 CFM versions. JET Performance offers remanufactured Rochester Quadrajet carburetors and tuning kits. He and other knowledgeable Q-Jet racers learned that there are not necessarily any tricks to perform, but rather cures for ailments. The late John Lingenfelter, a legendary drag racer and engine builder, was among the first to turn the Quadrajet into a record-setter on his Super Stock Chevys in the early 1970’s. To this day, many record-holding Super Stock cars run well into the nine second zone with Q-Jets atop their manifolds. 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. 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.Ī spread-bore by design, the 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. 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”. The long-discontinued carburetor remains popular with performance rebuilders and tuners. 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.
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