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How to Rebuild and Modify Rochester Quadrajet Carburetors How to Rebuild and Modify
Rochester Quadrajet Carburetors
by Cliff Ruggles
The Rochester Quadrajet carburetor was found perched atop the
engine of many classic GM and Ford performance vehicles. The
Q-Jet is a very capable, but often misunderstood carb. This book,
How to Rebuild and Modify Rochester Quadrajet Carburetors,
seeks to lift the veil of mystery surrounding the Q-Jet and show
owners how to tune and modify their carbs for maximum
performance. A complete guide to selecting, rebuilding, and
modifying the Q- Jet, aimed at both muscle car restorers and
racers. It includes a history of the Q-Jet, an explanation of how the
carb works, a guide to selecting and finding the right carb,
instructions on how to rebuild the carb, and extensive descriptions
of high-performance modifications that will help anyone with a
Q-Jet car crush the competition.
Click below to view sample
pages from each chapter
Chap. 1 - Quadrajet History
Chap. 2 - How Q-Jets Work
Chap. 3 - Carb Selection
Chap. 4 - Tools & Safety
Chap. 5 - Q-Jet Rebuilding
Chap. 6 - Performance Mods
Chap. 7 - Edelbrock Q-Jets
"Unlike some tech books you've probably seen, this one does a
good job on the photography, with all color photos shot with good
lighting, clear details, and clean backgrounds." -Musclecar
Enthusiast, October 2006, reviewed by Steve Statham
8-1/2 x 11"
Softbound
128 pages.
Approximately 300 color photos
Item: SA113
Price: $22.95
Click here to buy now!
This is a great book
anyone with a
Quadrajet will love!


History of the Rochester Quadrajet
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.
1966 Quadrajet carburetor 7026260
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.
Rochester 4GC compared to a Quadrajet
The carburetor on the
left is a Rochester 4GC,
used widely by General
Motors until the
Quadrajet (shown on the
right) was introduced.
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.

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.
A front inlet quadrajet compared to a side inlet model
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.
A Quadrajet manufactured my Carter Early plunger style fuel valve assembly
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.
Float fulcrum locations compated on different models of Quadrajet carburetors
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.
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.
Next


This has been a sample page from

How to Rebuild and Modify Rochester Quadrajet Carburetors How to Rebuild and Modify
Rochester Quadrajet Carburetors
by Cliff Ruggles
The Rochester Quadrajet carburetor was found perched atop the
engine of many classic GM and Ford performance vehicles. The
Q-Jet is a very capable, but often misunderstood carb. This book,
How to Rebuild and Modify Rochester Quadrajet Carburetors,
seeks to lift the veil of mystery surrounding the Q-Jet and show
owners how to tune and modify their carbs for maximum
performance. A complete guide to selecting, rebuilding, and
modifying the Q- Jet, aimed at both muscle car restorers and
racers. It includes a history of the Q-Jet, an explanation of how the
carb works, a guide to selecting and finding the right carb,
instructions on how to rebuild the carb, and extensive descriptions
of high-performance modifications that will help anyone with a
Q-Jet car crush the competition.
Click below to view sample
pages from each chapter
Chap. 1 - Quadrajet History
Chap. 2 - How Q-Jets Work
Chap. 3 - Carb Selection
Chap. 4 - Tools & Safety
Chap. 5 - Q-Jet Rebuilding
Chap. 6 - Performance Mods
Chap. 7 - Edelbrock Q-Jets
"Unlike some tech books you've probably seen, this one does a
good job on the photography, with all color photos shot with good
lighting, clear details, and clean backgrounds." -Musclecar
Enthusiast, October 2006, reviewed by Steve Statham
8-1/2 x 11"
Softbound
128 pages.
Approximately 300 color photos
Item: SA113
Price: $22.95
Click here to buy now!
This is a great book
anyone with a
Quadrajet will love!


Other items you might be interested in

Rebuild and Modify Carter Edelbrock Carburetors
How to Rebuild and Modify Carter and Edelbrock
Carburetors reflects the emergence of Edelbrock
carburetors as the predominant carburetors in the market
today. Author David Emanuel outlines carburetor types,
gives a thorough look at carb selection and carb function,
and offers detailed information on modifications, tuning,
and rebuilding Carter and Edelbrock carburetors. Also
features the history of Carter as well as the history of
the AFB and the AVS since the purchase by Edelbrock.
Rebuild and Modify Carter Edelbrock Carburetors
Price:
$ 22.95



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