The Rochester Quadrajet was last installed onto a Pontiac engine during the '81 model year, and most of those used during the final years of production were computer assisted. So it's unlikely that they'll be found in performance applications today, but what about the myriad conventional castings Pontiac used on several engines for the '67 model year and then exclusively from '68 forward? Still relatively plentiful, virtually any common casting can make an excellent high-performance unit once correctly modified.

If you read "Quadrajet Quotient" in the Dec. '06 issue of HPP, you know that the different Pontiac castings used over the years share similar primary-airflow characteristics and Rochester used the secondary air valve to limit total airflow for most applications. Increasing air-valve angle can maximize total airflow, subsequently improving full-throttle performance in applications where an engine is airflow-limited, but what happens to idle quality when placing a stock 301ci casting onto a heavily modified 455?

An original casting is calibrated to supply a sufficient range of idle fuel for its intended application, but even the simplest modifications to an engine can change those requirements. Enlarging a few small restrictions by drilling can increase available idle fuel and transform the Quadrajet into an excellent, low-buck alternative that can satisfy the needs of most any combination. So follow along as we improve the idle quality and off-idle and low-speed street manners of a '67 GTO. You may find that a small amount of tuning can pay off in a big way for your Pontiac, too.

Carburetor Basics
The carburetor plays a critical role in engine operation. It must provide the correct mixture of atomized fuel and air for maximum performance in every operating condition, and it uses three specific fuel circuits to accomplish this; idle, primary (or main), and secondary. The primary and secondary fuel circuits control moderate-to-heavy part-throttle and full-throttle performance, respectively. Either can be altered by metering jet and/or rod changes, but the idle circuit of a typical Quadrajet is much more complex.

A Quadrajet's idle circuit was designed to maximize idle and low-speed operation, but the circuit continues to operate as engine speed increases, also affecting the primary and secondary calibrations. The circuit consists of a series of fixed restrictions within the main body and a pair of mixture screws located on the throttle body. Under normal operation, an engine draws fuel and air through the primary barrels, but at idle and low-speed, air velocity isn't great enough to initiate the main fuel circuit. Sufficient fueling from the idle circuit is required.

During such conditions, fuel is drawn from the main fuel well through calibrated idle tubes in the main body. As the fuel travels towards the engine, it mixes with a small amount of air drawn from calibrated upper air-bleeds found in either the air horn or main body. The air/fuel mixture, which is comprised mostly of fuel, passes through down-channel restrictions in the main body, where it is discharged into the engine through tapered mixture screws located below the throttle blades, and mixes with a larger volume of air that passes around the primary throttle blades.

Adjusting the mixture screws can increase or decrease the amount of available idle fuel entering the engine. An adjustment screw on the main body controls actual idle speed by changing throttle-blade angle, subsequently changing the amount of air passing though the primary barrels. It is this delicate balancing act that produces a strong, stable idle that allows the engine to operate solely off of the idle circuit.

Some larger-displacement and/or high-output engines may need a greater volume of air than the throttle blades can pass during normal idle conditions. This can sometimes require excessive throttle angle, which allows high-velocity air to pass through the primary barrels and can lead to a number of issues, including erratic idle quality with little control or even premature actuation of the main fuel circuit, often referred to as nozzle drip.