PREFACE

The motivation to provide the information contained in this book came from many sources. My friends and fellow enthusiasts encouraged me for many years to make more information available. As fuel injection went into use across the board in 1988, I dismissed putting anything into hard copy and concentrated efforts on building carbs. The Internet made me realize that there are literally thousands of folks in need of assistance and there simply wasn’t enough time to help each and every one of them despite my best efforts. The answer was to publish the following material. I hope the information will be beneficial. The information included in this publication comes from countless hours of building, tuning, and testing Quadrajet carburetors. I have included a lot of ideas that I’ve developed over two decades of working with these carburetors and tried to present it in an easy-to-understand format.

INTRODUCTION

Quadrajet carburetors began showing up on factory production General Motors vehicles in 1965 on Chevrolet 396 engines. Due to ever-tightening emission standards, the auto-making industry was being forced to clean up the amount of pollutants exiting the tailpipes of production vehicles. At the same time, the end of the decade brought with it the peak of the muscle car era. Buyers demanded performance, and the automakers were in a tight race to produce the fastest and most appealing vehicles. Mustangs, Camaros, 442s, GTOs, Roadrunners, and many others showed strong sales and ruled the streets.

These events had the engineers scrambling to produce powerful cars with large-displacement engines, and, at the same time satisfy the emission restrictions mandated by the EPA. Several engine design changes were employed. Chrysler and Pontiac modified the shape of the combustion chambers in many of their engines in 1968. Compression ratios were lowered in 1970, and again dramatically across the board in 1971. Power production fell hand in hand with the drop in compression. Rumors of oil shortages were to follow, and EGR valves and air pumps became standard equipment. Fuel prices began to soar, the reign of the muscle car was at an end.

For General Motors, the Quadrajet carburetor (Q-jet) was a key player during this period. It went through several design changes between 1967 and 1974, and then it was given a complete makeover in 1975–1976. Each change had a purpose, and most of them hand nothing to do with high performance. The competition between the automakers had quickly gone from making the most power and fastest cars to having the least amount of hydrocarbons exiting the tailpipes. The Q-jet survived the battles, while the factory Holley, AFB, AVS, and even the Thermoquads eventually disappeared. The Q-jet made it through, and even after computer engine management systems became standard equipment, the Q-jet stayed in production. By 1988, GM had fuel injection in place across the board, and we saw the very last of the computer-controlled carburetors disappear. Like it or not, our fuel and spark curves were now controlled by electronic components; carburetors became like dinosaurs.

Even though the muscle car era came and went, the interest in high-performance vehicles never died. In fact, it is probably stronger now than ever. Countless enthusiasts spend hundreds of hours each year building, tuning, racing, and enjoying their high-performance vehicles. The interest in the Quadrajet carburetor has become increasingly strong. A high percentage of hobbyists demand that ALL of the factory equipment be used for the restorations, and nearly as many love to use the factory parts as part of their high-performance vehicle package. The Q-jet carburetor has met considerable resistance when employed as a high-performance part. Yet, there is no doubt that they can be made to work. Just attend any IHRA- or NHRA-sanctioned event and watch the Stock and Super Stock cars yank the front wheels into the air and set new records nearly every season. Even so, from the hobbyists’ point of view, the Q-jet is simply too complicated and results are simply too unpredictable, even after spending considerable manhours attempting to make one work correctly.

Well, we are going to change all that! In this book, all of the secrets of the Q-jets are revealed. We’re going to go way beyond just jet and rod changes. Each system is broken down in great detail. The flow of air and fuel through the carburetor is described, so the reader not only knows where and how to make modifications, but why they are making them. Specific modifications are described and the intended results outlined. We’re going to take the guesswork out of correctly setting up your carb for high-performance use. The different models are also discussed, as are factory changes and why they were made. We’re going to guide you through building a top-performing carburetor exactly for your application, and most of it can be done with a very minimum investment of tools and equipment. If you ever wanted to know how the Stock and Super Stock racers do it, and how those nearly 4,000-pound factory vehicles that sport a Quadrajet carburetor find their way into the winner’s circle at your local track, keep reading, because all the secrets are found in these pages.

The Quadrajet carburetor is in no uncertain terms an amazing creation. Its purpose in life was to provide a highly efficient fuel-metering device using small primary side bores and huge secondaries for outstanding full-throttle performance. Almost anyone who mingles in high-performance circles has at one time or another had an experience with a Q-jet carburetor. Sadly, in many cases these associations have met with less-than-satisfactory results.

After spending over 25 years rebuilding, recalibrating, modifying and testing Q-jet carbs, it’s now time for me to make the results of these efforts readily available. We are going to take you step-by-step through the entire Q-jet carb, system-by-system. Different models are discussed, as are parts interchangeability, high-performance modifications, and specific modifications needed for specific applications. The modifications are not only being explained in great detail, but most can be made without going broke from buying a bunch of special tools. Most of the modifications don’t even require extensive skills or vast experience with carburetors.

CHAPTER 1

HISTORY

The Rochester Quadrajet was a standard production four-barrel carburetor used by every General Motors division for nearly 20 years. Although it never deviated far from its original basic design, the Quadrajet underwent several modifications over the years, proving its versatility over a wide variety of applications. Some of these changes are covered in this chapter. But those that are most relevant when preparing a Quadrajet for high-performance use are covered in greater detail in later chapters.

It is important to understand the basic Quadrajet design and reasoning behind the changes made throughout its production span. We must first realize that from a practical standpoint, most Quadrajets were never intended to be high-performance carburetors. General Motors produced vehicles for a very broad customer base, and each division developed its own engines. Since many different engines were offered in a wide variety of vehicles, nearly every carburetor was calibrated for the specific application. With ever-increasing federal emission standards, manufacturers were forced into making their engines as efficient as possible, which led to future changes. But to better understand the Quadrajet, we must start right from the beginning.

Shown here is a 1966 Quadrajet, carburetor number 7026260. This basic design was typical of very early production carburetors. Carburetors produced in later years would appear similar externally, but many internal changes were made to make them more reliable.

The Rochester Products division of General Motors was located in Rochester, New York, and had built single- and multiple-bore carburetors for GM vehicles since 1949. The AC Delco division, however, issued service information and distributed the carburetors and service components. Rochester’s 4-barrel casting known as the 4G had been one of GM’s standard production carburetors from the mid-’50s until the mid-’60s. But as economy and emissions concerns grew, Rochester responded with an entirely new 4-barrel carburetor, the Quadrajet. It boasted small primary bores for maximum throttle response and fuel efficiency, and larger secondary bores to meet the overall flow demands under full-throttle conditions. The Quadrajet was designated the 4M series. This would be the basic casting from which all future variations evolved.

Chevrolet was the first and only GM manufacturer to use the 4M Quadrajet in 1965 on certain V-8 applications. The 4G 4-barrel, however, was used on other Chevrolet V-8 applications, while radical, high-performance applications retained the Holley 4-barrel. Two different Quadrajet product numbers were produced—the 7025200 for 396-cubic-inch engines with automatic transmissions, and the 7025201 for 396-cubic-inch engines with manual transmissions. As the model year progressed, product numbers 7025220 and 7025221, with what apparently revised choke settings, superseded the previous numbers. The product identification number was stamped into a circular disc located on the driver’s side of the main body. Identification is discussed later.

The Quadrajet saw expanded use onto a wide variety of 1966 Chevrolet V-8 car and truck engines with displacements that ranged from 327 to 427 cubic inches. It even saw use on select overhead-cam 6-cylinder applications. Buick also selected the Quadrajet for its 400 and 425 cubic-inch engines in 1966. Oldsmobile followed suit. Cadillac and Pontiac were not as quick to change, but by 1967 they too had begun installing the Quadrajet on most of their 4-barrel applications. With its growing popularity and efficient design, the Quadrajet was the most widely used production 4-barrel carburetor on GM-produced engines during that time. And though other carburetors such as the Rochester 4G, the Carter AFB, and the Holley 4-barrel were used on certain engines, the Quadrajet quickly became a benchmark for other 4-barrel carburetor manufacturers.

The 1968 model year marked the first nationwide federal standards for specific pollutants. This not only brought on internal engine changes, but many GM engines received components that controlled the spark advance and fuel curves throughout the entire operating range. And because of its efficiency and excellent balance of economy and performance, the 4M Quadrajet became the standard production 4-barrel carburetor for all GM manufacturers including GMC, Checker Cab, and the Marine division. The Rochester 4G was phased out as the Quadrajet replaced everything except the Holley 4-barrel, which continued on certain Chevrolet applications through 1972.

The carburetor on the left is a Rochester 4GC, used widely by General Motors until the Quadrajet (shown on the right) was introduced.

With the Quadrajet used extensively by all GM divisions, each manufacturer required certain external characteristics that matched its under-hood routings. The most common difference many hobbyists are most familiar with is the forward-facing fuel inlet as opposed to those that are side facing. And since the Quadrajet was used on such a wide variety of engine sizes from each respective automobile manufacturer, separate fuel-metering circuits and a variety of vacuum sources were also required and each carburetor was given a specific casting number.

One of the first major changes to the Quadrajet design was making it available in both front- and side-inlet models, shown here. The front inlet is shown on the right; side inlet is on the left.

Demand from the divisions had increased to the point that Rochester could not meet the volume requirements. To prevent production delays, GM approached the Carter Corporation about producing Quadrajets in addition to Rochester. The Carter-built units are nearly identical to the Rochester-produced Quadrajet—the only differentiating characteristic is Quadrajet by Carter cast in place of Quadrajet by Rochester on the side of the main body. Since Carter produced Quadrajets well into the 1970s, they are not necessarily uncommon. Either would make an acceptable starting point for performance modifications.

Not unlike anything designed from a clean slate, the early Quadrajets were plagued by several small problems, such as the plunger-style fuel valve and the secondary-air-valve dash pot assemblies. But they were improved upon and increased overall function and reliability in subsequent years.

In the late 1960s, Carter was contracted to produce Quadrajet carburetors to help keep up with production. The Carter units were manufactured under contract and clearly state MFD. By Carter Carburetor for GMC as seen in the bottom casting.

Very early Quadrajet carburetors used a plunger-style fuel-valve assembly. These were prone to trouble and disappeared in 1967.

Many of these are discussed later in the book. But the first notable changes occurred for the 1969 model year. A smaller float with a relocated fulcrum increased float-bowl volume and provided more pressure on the fuel needle to better control the fuel level. Though these types of changes were typically implemented in all the Quadrajets produced, the Oldsmobile and marine applications retained the earlier-style float for several years.

Another series of changes occurred in 1971. Plastic caps were placed over the idle mixture screws on the base plate of certain Quadrajets. These were used to limit travel and prevent grossly inaccurate adjustments, which could affect emissions. Internal metering changes were also required, with General Motors implementing a maximum compression ratio of just 8.5:1, which applied to each division for the 1971 model year. This was not only an attempt at reducing unburned hydrocarbon and oxides of nitrogen emissions, but also allowed the engines to operate on low-lead or unleaded fuels. The federal government had proven that lead particulates emitted from automobile tailpipes were finding their way into environmental areas such as water streams and farming soil. These negative effects on the environment and humans prompted governing bodies to impose limits on the amount of lead present in fuel. Eventually it was removed entirely.

Leaded gasoline had been available for several decades prior. The lead was added in an attempt to increase the overall octane rating of the fuel but was later found to also improve the life of the exhaust valve seats. Since lead does not burn, it typically left behind a powdery residue, which coated the entire combustion chamber, including the exhaust valve and exhaust valve seat as the gasses exited. As lead was removed, the extreme heat of the exhaust gasses passing over the valve seat and the rotating action of the valve itself generally wore untreated cast- iron seats more quickly, especially with an engine that was frequently under heavy load. It was determined that the lead acted as a lubricant, preventing direct contact between the valve seat and the valve and reducing the wear process.

Moving the float fulcrum forward for 1969 (carburetor on right) models was one of the first major design changes for the Quadrajet. The early-style float arrangement continued in production well into the 1970s for various applications.

Many early models featured plastic caps over the idle-mixture screws to discourage tampering. It’s quite rare to see any in place these days. They were one of the first items removed from the carburetor during basic engine tune-ups.

In 1971 Pontiac used a special version of the Quadrajet lacking the outer booster rings. The idea was to provide additional airflow, but these carburetors lacked the part-throttle efficiency of standard units and were difficult to get through emission testing. They were discontinued by the 1972 model year.

Shortly after lead restrictions were enacted, many GM divisions began taking measures to reduce wear by induction-hardening the exhaust-valve seats on their cylinder heads. This process incorporates heating the valve seat to approximately 1800 degrees F for a short time with a coiled wire fixture. As the seat cooled, the result was an electrically inducted, hardened seat that extended about .050-inch below the surface. This greatly increased exhaust valve seat life and improved engine performance. The compression ratio loss took its toll, but performance was still strong from all the GM manufacturers. While the others may have approached improving lower-compression performance in a different manner, Pontiac and Buick both compensated by addressing the Quadrajet.

Pontiac modified the basic 4M Quadrajet casting by removing the outer