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.
To combat said situation, Rochester incorporated a fixed bypass that allows a specific amount of filtered air to enter the engine without passing through the primary barrels. The bypass acts similarly to a controlled vacuum leak and allows for a proper air/fuel mixture while maintaining greater control from the idle-speed adjustment screw. However, no matter how much air an engine ingests at idle speed, the air must be combined with sufficient fueling to attain proper idle quality.
What many hobbyists fail to consider is that each carburetor was calibrated for its original application, and the amount of available idle fuel is directly related to the range of requirements for that combination. Simply stated, installing a unit originally designed for a small-displacement, smog-era application onto a heavily modified performance engine may not contain sufficient idle fuel to always yield positive results, but a few calculated modifications found in a newly released book may no longer make that a great concern.
Modifying The Quadrajet
In the past, hobbyists have referred to the Quadrajet by a number of disparaging names, and most of these can be attributed to poor idle and off-idle characteristics. Until recently, only a handful of gifted carburetor tuners had the skills and knowledge required to properly prepare a common Quadrajet casting for a specific application. S-A Design's recent release, How to Rebuild and Modify Rochester Quadrajet Carburetors, by Cliff Ruggles of Cliff's High Performance in Mount Vernon, Ohio, has brought carburetor tuning into our own garages.
Local hobbyist Alan Fanning recently approached us about the poor operating characteristics of his '67 GTO. The car contains many of its original components, including the numbers-matching 400ci block, No. 670 cylinder heads, cast-iron intake manifold, Turbo-400 transmission, and 2.93 rear axle ratio. The engine has, however, been rebuilt and now displaces 406ci and has been fitted with an aftermarket 068-type camshaft featuring 216/228 degrees of duration at 0.050-inch. Though the carburetor is not the original No. 7037262 unit, it is a '67 GTO manual-transmission casting (No. 7037263), so its internal fuel calibration should be close to its automatic-trans brethren.
Fanning complained of hard starting, poor idle quality and fuel economy, and an overall lack of throttle response. Since we had previously calibrated the GTO's points-type distributor on our distributor tester, we directed our attention towards the carburetor. It had been "rebuilt" in the past, but all other attempts at tuning the unit proved futile. Knowing that it likely needed serious attention, we purchased a complete Quadrajet rebuild kit from Cliff's High Performance and made arrangements with Fanning to leave his GTO with us for a weekend.
Upon receiving the GTO, we recorded a preliminary manifold vacuum reading of roughly 8 inches. We noted poor throttle response and an over-rich condition being emitted from the tailpipes. Additionally, the mixture screws had no real effect on idle quality, and engine speed increased when a vacuum hose was removed, indicating that the engine wanted more raw air than the carburetor was supplying. A subsequent test drive revealed that the engine also wanted to stall as we approached a stop sign or stoplight.
With all indications pointing at the carburetor, we quickly removed the Quadrajet from the engine, placed it on the bench, and began disassembly. A close inspection revealed that the casting was very clean and free from any warpage, but we also found that Fanning's Q-jet was still equipped with its original 0.070-inch primary jets, 0.039-inch primary metering rods, and 0.041-inch secondary metering rods, and the throttle body was not equipped with any bypass air.
Prior to performing any modifications, we located and recorded the measurements of the upper and lower idle-air bleeds, the down-channel restrictions, and the idle tubes using a complete set of numbered drill bits. Although measuring the restrictions was straightforward, we did have to remove the idle tubes from the main body. This can be a tricky process, but Ruggles' book offers a few excellent suggestions to ensure safe removal.
Original Calibration Upper Idle Air-Bleed 0.039 inch Lower Idle Air-Bleed 0.070 inch Idle Tube 0.031 inch Down-Channel Restriction 0.040 inch Throttle-Body Bypass Air None Primary Metering Jet 0.070 inch Primary Metering Rods 0.039 inch Secondary Metering Rods 0.410 inch "CE" codeWith the original calibration recorded, we used the guidelines found in the book to determine which calibration was best suited for the characteristics of the 400ci we were working with. We then carefully drilled each restriction and added bypass air to the throttle body. We also used several components from the rebuild kit to properly prepare the Quadrajet for service including a pair of 0.073-inch primary metering jets, and we replaced the original 0.039-inch primary rods with a pair of 0.043-inch units that we had available.
Initial Calibration Upper Idle Air-Bleed 0.052 inch Lower Idle Air-Bleed 0.070 inch Idle Tube 0.037 inch Down-Channel Restriction 0.046 inch Throttle-Body Bypass Air 0.085 inch Primary Metering Jet 0.073 inch Primary Metering Rods 0.043 inch Secondary Metering Rods 0.410 inch "CE" codeAfter priming the carburetor for initial start up and setting the mixture screws approximately 2 1/2 turns out from fully seated, we were rewarded with an engine that fired immediately. It idled much smoother, and the exhaust was much cleaner. The idle-mixture screws had a noticeable effect on idle quality, and manifold vacuum increased towards 12 inches. However, we found that removing a vacuum hose still caused engine speed to increase slightly, indicating that the engine might benefit from additional bypass air.
With the carburetor removed from the intake manifold for disassembly, we proceeded to enlarge the bypass restrictions 0.015 inch to a total of 0.100. Wanting to prevent any fuel-related issues, we also increased the diameter of the down-channel restrictions to 0.052 inch. Together, these mods provide the engine with more air and fuel at idle, yet allow for full control over idle speed and quality with the idle-speed and mixture screws.
Final Calibration Upper Idle Air-Bleed 0.052 inch Lower Idle Air-Bleed 0.070 inch Idle Tube 0.037 inch Down-Channel Restriction 0.052 inch Throttle-Body Bypass Air 0.{{{100}}} inch Primary Metering Jet 0.073 inch Primary Metering Rods 0.043 inch Secondary Metering Rods 0.410 inch "CE" codeThe small changes we made were immediately detectible. Trimming the mixture screws pushed manifold vacuum to a maximum of nearly 15 inches at 950-1,000-rpm idle speed, and placing the automatic transmission into gear didn't dramatically affect idle quality nor did the engine want to stall when coming to a stop. Throttle response noticeably improved, and the exhaust tone not only gained a pleasant "tuned" sound, it also lost the persistent odor of unburned fuel. But no matter how well it idled in the garage, the real test would come once it was on the street.
Backing the GTO from the garage, we found that less throttle angle was required to get it moving. The vehicle accelerated noticeably smoother, and there was no detectible transition between the idle and primary circuits. Heavy acceleration from a stop revealed the engine pulled smoothly through the rpm range, and although we had not altered the secondary circuit, the engine seemed to have seamless transitions between the various circuits.
When Fanning returned to claim his GTO, he immediately noted how easily the Pontiac started. After his return trip home and a few weeks of normal driving, Alan says, "One of the very nice things is that it comes off idle very smoothly without any hesitation or bog at all. And it goes down the road at 70 mph nicely. When you step into it, the torque and the tall gears come into their own, and it reminds you how great of a highway car it is." He also reported a sharp fuel-economy increase on a recent road trip of several hundred miles.
What We Learned - After spending a weekend tuning the GTO, it became apparent that this Quadrajet's original calibration wasn't best-suited for the modifications this 400ci engine contained. With the assistance of How to Rebuild and Modify Rochester Quadrajet Carburetors, we successfully richened the idle circuit, which not only allowed us to lean a portion of the primary circuit but also rid the carburetor of its "nozzle-drip" condition. We also learned that each application is slightly different, and similar combinations cannot be tuned identically. In the end, a slight amount of trial and error may provide the best results, but when dealing with carburetors, keep in mind that a change as minute as 0.001 inch can have dramatic effects.
Conclusion
Over the years, we have heard from those who detest Quadrajet carburetors due to the unsuccessful tuning attempts they have made in the past. But after seeing how well the 400 in Fanning's GTO responded to a few simple idle circuit modifications and the resultant increase in engine efficiency, our example further proves the Q-jet's versatility when modified correctly. We also hope that the simplicity of these modifications proves that no casting should be discarded because of its original application, and any unit can contain the potential for high-performance use.
Although idle-circuit modifications are not something most beginners want to tackle, we are confident that after reading How to Rebuild and Modify Rochester Quadrajet Carburetors and then applying the idle-circuit modifications, any hobbyist can achieve similar results. With a little practice, you too can become a skilled carburetor tuner with the ability to turn a swap-meet or salvage-yard bargain into a high-dollar performer for your Pontiac with minimal out-of-pocket expense.
Special thanks to Jim Hand for his contributions to this article.
