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We ship world wide.
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Selecting the Right Carburetor
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In previous chapters we have covered the history and evolution of the Quadrajet and the basic
systems and how they work. Before moving on into rebuilding and high-performance modifications,
you need to make sure that the carburetor you have chosen meets your basic requirements so
your finished product works and performs as expected.
The good news is that the basic design of the Quadrajet stayed the same throughout the years of
production. The Quadrajet would receive numerous minor changes. Some of these changes were a
direct result of ever-tightening emission standards. Others were to make the basic design more
flexible and reliable. Most of the changes were for the better, and as you will see, the later
production units sport a lot of very good features, making them an excellent starting point for a
high-performance carburetor.
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These two 1969 Pontiac carburetors
look very similar. The carburetor on
the right is a factory high-performance
carburetor used on 400ci Ram Air
engines. The carburetor on the left
was used on the standard 400 engine
the same year. Both have slashed
front vent tubes. The “270” Ram Air
carburetor used a slightly different
calibration than the standard
production “262” carburetor; otherwise
they are nearly identical.
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The use of the Quadrajet through its years of production was so broad that countless used
samples are currently available and will be for many, many years. It has long been thought that the
only carburetors worth working with were the rare and highly coveted factory high-performance
carburetors. This has had countless enthusiasts searching the salvage yards and swap meets for
years, drying up just about all of these units. The good news is that the factory “high-performance”
carburetors were only slightly reworked standard production units. This means that, armed with the
right knowledge and abilities, we can rework our standard production Quadrajet to virtually any
level of performance needed.
The carburetor you choose to work with still has to meet some basic requirements for the
application. There are several minor things to consider, such as fuel inlet location, CFM capability,
linkage, vacuum fittings, etc. We also have to look at the year of production and overall condition of
the carburetor. In many cases it is more cost-effective to choose a later production unit in better
condition than to spend a lot of hours working with a well-worn early model.
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What Are the “Best” Quadrajets?
I have been asked this question countless times. The most accurate and direct answer would be
the factory high-performance carburetors. The very best factory Quadrajets would be the 1973 and
1974 Pontiac 455 Super Duty carburetors.
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In 1973 and 1974 Pontiac used a
special casting with larger primary
bores for their Super Duty 455
engines. These carburetors have
excellent idle-fuel calibrations along
with very generous fuel curves. They
can be quickly identified by the large
slashed front vent tube. They are
excellent high-performance units, but
extremely rare and highly coveted by
restorers.
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They were among the first to use the larger castings. They also had an excellent secondary fuel-
enrichment system and generous idle and off-idle fuel settings. These particular carburetors also
used the hot-air-style chokes and can easily be converted to electric choke, making for easy
retrofitting on most any application. They work quite well in near stock form on most any high-
performance application. The problem with them is that they were produced in very limited
quantities; the Pontiac Super Duty Trans Am was quite rare. Even service replacement Super Duty
carburetors are rare and bring high prices.
Next in line would be the 1971–1974 Buick carburetors delivered on the 455 engines. These
carburetors also used the larger castings, yet were divorced-choke. They make excellent
carburetors for use on any early factory intake that had a divorced choke set-up, particularly where
the user has swapped in a high-performance or large-CID engine. The large-CFM Buick
carburetors are relatively common and can be purchased at reasonable prices.
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Many 1971–1974 Buick carburetors also
featured the larger castings. These were the
only divorced-choke, large-CFM castings
produced. They can be easily identified by the
slashed front vent tube and large “hook” on
the throttle linkage. They make excellent
carburetors for any divorced-choke
application; however, they lack any provision
on the throttle linkage for transmission cables
and return springs behind the carburetor.
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Following closely behind the large-CFM Pontiac Super Duty and 455 Buick carburetors would be
the factory Pontiac 455 HO carburetors released in 1971. These were the only factory Quadrajets
to not use the outer booster rings surrounding the main nozzles. They were not units that had the
rings removed; they were actually special castings where the inner booster ring would extend
downward nearly to the bottom of the casting.
This was probably done to help improve nozzle efficiency lost from not using the outer rings.
However, these units proved to be very inefficient off idle and were discontinued after a single-year
production run. As with the 1973 and 1974 Super Duty carbs, the 1971 HO carburetors were
produced in very limited quantities and are highly coveted by restorers.
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1975 APT Carburetors
Some 1975 and 1976 Quadrajets also used a second power piston and single jet/rod in the main
body. Although these carburetors have the same potential as any other late-style Quadrajet, they
are more labor intensive to recalibrate correctly. Due to the increased effort, it may be a better idea
to simply get another carburetor that doesn’t use this system. The single APT carburetors have
internal passages that route additional fuel beneath the main jets. The factory pressed a single jet
into the main body of the carburetor and added a single metering rod for fine external metering
control. The problem with this particular design is that raising the metering rod not only adds fuel at
part throttle, but does so at heavy and full throttle. This would have the engineers compensating for
the increased fuel with main system calibration. I’m sure this is not exactly what they intended, and it
met with mixed results as the design was only used for a brief period, then upgraded with the later
APT that raised the power piston only.
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New castings showed up in 1975
featuring an early APT system.
Located in the right front corner of the
carburetor is a brass cup plug
covering an adjustment screw. Turning
the adjustment screw provides fine
control of the part-throttle mixtures
externally.
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The later design raised only the power piston and used metering rods with a tapered second
section. This provided very fine control of the part throttle air/fuel ratios without affecting heavy and
full throttle metering. This later design was used throughout the remaining years of production for
non-computer managed carburetors.
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In 1976 the APT adjustment screw was
relocated to the center of the casting
in front of the choke housing. An
aluminum plug is driven into the
access hole for the APT adjustment to
prevent tampering. Here, we’ve raised
the plug slightly for the photo.
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Previous | Next
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This has been a sample page from
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How to Rebuild and Modify
Rochester Quadrajet Carburetors
by Cliff Ruggles
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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.
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Click below to view sample
pages from each chapter
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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
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"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
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8-1/2 x 11"
Softbound
128 pages.
Approximately 300 color photos
Item: SA113
Price: $22.95
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Click here to buy now!
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This is a great book
anyone with a
Quadrajet will love!
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Holley Carburetor Handbook
The Holley Carburetor Handbook explains the basics of fuel
inlet, idle, main metering, power valve and metering,
accelerator pump, secondary and choke systems on Holley
carburetors. It also covers carburetor repair and adjustment.
Bonus section describes how to choose a carburetor based on
engine size and volumetric efficiency. More than 100 photos,
charts and drawings show you how to do it right the first time.
This is a must have book for anyone working on the Holley
model 4150 or 4160 Selection.
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Price:
$11.95
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Rebuild and Modify Carter / Edelbrock Carburetors
How to Rebuild and Modify Carter/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. Contains more than 300 color photos,
illustrations, and diagrams.
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Price:
$
22.95
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Super Tuning and Modifying Holley Carburetors
Super Tuning and Modifying Holley Carburetors shows you
how to select, install, tune, and modify all popular Holley
performance carburetors. This book gives a detailed view of
basic carburetor functioning, modifying for performance
applications, custom tuning for street, racing, off-road,
turbocharging, economy, and other special uses.
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Price:
$
18.95
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Payment, Shipping & Sales
Tax: Iowa
residents must pay 7% sales tax. Items usually ship
within one
business day of receipt of payment! Standard shipping is a flat rate of
$4.95 to
anywhere in the United States with USPS Media Mail.
Priority Mail shipping is available for an
additional $2.95, or
$7.90 for shipping. Shipping is combined and discounted for multiple item
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$1.95 for each additional item. For
purchases of 3 or more items
shipping is automatically upgraded to Priority for no additional
charge! We offer world wide shipping and ship to Canada and Mexico
with USPS Priority Mail
International for $11.95, and to most
locations in Europe, Australia, Asia, Japan and South
America for
$14.95. Satisfaction is Guaranteed. Our store has a NO HASSLE RETURN
POLICY
within 7 days of purchase.
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