How well do you know your Rochester Quadrajet?
By Rocky Rotella
The Rochester Quadrajet was the primary 4bbl carburetor used on nearly every General Motors vehicle from about 1967 through the mid to late 1980’s. Millions were produced and installed but very few were properly calibrated for optimum performance from the factory. Since most people do not understand all functions a carburetor must perform, we’ll discuss the basic operation of the Quadrajet and what it means to your vehicle’s performance.
In Quadrajet fuel metering, there are two separate circuits to feed the engine’s fuel needs; the primary circuit and the secondary circuit. While these two circuits are adjustable and can be calibrated to meet engine requirements, there is also an idle circuit, which has no bearing on performance whatsoever. The idle circuit consists of a non-adjustable fuel/air bypass that allows the carb to idle steadily without tipping into the primary circuit. There are a pair of mixture screws, one located in front of each venturi on the base plate that allow the idle fuel/air ratio to be trimmed but overall adjustment is limited. The Q-jet will accept plenty of radical combinations before the idle/air passages require enlarging for even greater air capacity. In most cases, the stock sized idle/air bleeds are more than sufficient.
The Quadrajet primary circuit consists of a pair venturi (or barrels) that are small enough to produce excellent throttle response, low speed street manners and fuel economy. An accelerator pump provides an initial shot of fuel to reduce/remove the resulting bog when the primary throttle blades begin to open and the carburetor transitions from idle circuit to primary circuit. The accelerator pump is the “shaft” that protrudes from the driver’s front corner and bobs up and down when the throttle blades are opened/closed. Once underway and running strictly on the primary circuit, a pair of replaceable, sized metering jets and a pair of replaceable, sized metering rods meter the primary fuel curve. Primary metering jets are sized in increments of .001” and are often referred to by their respective sizing. For example, a “74 jet” actually measures .074”. Additionally, primary metering rods have a tapered shape that leads to the “power tip”. While nearly all primary rods have a power tip of .026”, the taper into the power tip is how they are measured. Like the primary jets, they are sized in .001” increments and often referred by their measured taper where 43 (for example) means .043”. The .043” is the taper which leads to the .026” power tip.
When figuring a metering curve, primary jets and primary rods work in conjunction with a sprung loaded power piston. The jets are located in the base of the float bowl and fuel passes through them to feed the engine. The metering rods hang on the sprung power piston and bob up and down when spring tension is overcome by manifold vacuum. When manifold vacuum is high like that at idle or part throttle cruise, the spring is compressed, the power piston is in the down position and the metering rods are fully seated in the jets offering their .043” (going back to the examples above) metering to a .074” jet. .074” minus .043” equates to .031” fuel passing through each jet when the power piston is down. Vacuum reaches its highest point when the engine is most efficient and needing the least amount of fuel.
On the opposite end, when manifold vacuum is reduced and spring tension is higher than the vacuum forces applied to it, the power piston moves to the “up” position. The metering rods then ride high in the jets on their .026” power tip letting more fuel pass through when more fuel is needed. Typically, low vacuum conditions arise when the engine is under more load such as acceleration or climbing a grade. Basically, when the throttle is depressed and the throttle blades open further, vacuum continues to drop and the more fuel the engine needs. Power piston springs are specifically calibrated and replaceable and in the case of a more radical combination where manifold vacuum is low to begin with, lower tension springs are available to match the lower vacuum readings. Because manifold vacuum is never constant when driving, the metering rods continually bob and re-meter the fuel that enters the engine. If too large of jet or too small of rod is selected, overly rich or overly lean conditions can exist. If the power piston spring doesn’t offer correct tension for the specific combination, running rich or lean can again exist from too much or too little spring tension and the position of the rods relative to the engine’s fuel demands.
The secondary side of a Quadrajet is vacuum operated meaning the secondary air valve will only open as far as the motor actually requires. The secondary venturi are large enough that overall WOT (Wide Open Throttle) performance is satisfactory since the motor tells the carb what it needs and when it needs it. The secondary air valves are the sprung loaded larger plates on the top of the carb. Tension is adjustable and can be set for the plates to open quicker or slower depending on how it has been set. Additionally, a vacuum break acts as a timed release and connects to the secondary air valve via linkage. The vacuum break, or pull-off as they are commonly called is under tension very much like the primary power piston. Manifold vacuum retracts the pull-off and an inline restriction controls release rate when vacuum drops. This function allows a smooth opening of the secondary air valve creating a smooth transition from primary to secondary circuit rather than plopping open creating a bog. On more radical combinations where secondary response time is critical for racing, it becomes very difficult to correctly set for optimum throttle response. Manual transmission cars are more susceptible to improperly set secondary release time since many drivers lift off the throttle to shift into the next gear causing the secondaries to reopen when back on the throttle aggressively.
The secondary air valve has a shaft mounted cam that the secondary metering rod hanger rides on. When the secondaries begin to open, the hanger follows the profile of the cam and lifts the secondary metering rods allowing fuel to pass through the secondary metering jets. The secondary jets, which are located on the float bowl floor, are pressed in place and non-replaceable. Because secondary metering rods come in such a variety of sizes, secondary jet size is not an issue. Secondary rods are also sized but in increments of .0001” but unlike their primary metering counterparts, they are referred to by the differentiating tip sizes and are give two letter designations. Each rod has a specific taper and specific tip length which should be factors to consider when selecting a secondary rod for a specific application. Rods may exist with identical tip size but they will have a different taper and are subsequently given a different letter designation. There is no pattern between letter code and tip diameter. Secondary rod tip diameter can range from as thin as .0297” to as thick as .1047”. Obviously thinner rods will allow more fuel to pass through the secondary jet than a thicker rod. A longer tip will allow fuel to pass at a lower secondary air valve angle or when the secondary air valve is not completely open. Because the secondary venturi size is much larger than the primary, many times drivers feel a surge as the secondary barrels kick in. Quadrajets are notorious for the 4bbl moan that so many enthusiasts love.
Carburetors are at best a controlled vacuum leak with fuel mixed in to create three specific curves so the engine will run on idle, primary and secondary circuits. It is not very scientific when compared to a modern fuel injection unit yet many people overlook the carburetor’s job. Most people are often intimidated to look at or modify a carburetor any more. There is much more than meets the eye when you realize requirements that a carburetor must achieve. Many people do not realize how intricate and precisely built and tuned a carb must be in order to get the best compromise of good drivability, good economy yet still perform the way we want. The Quadrajet offers the best compromise of the three regardless of what anyone may say!