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Buy with confidence!
If for any reason
you're not
completely satisfied
with an item, simply
return it within 7
days and the
purchase price will
be refunded.
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We ship world wide.
All international
orders must be paid
online. Checks or
money orders drawn
on non-US banks will
not be accepted.
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Quadrajet Performance Modifications
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Now that you’ve learned how the Quadrajet works, how to correctly rebuild it, and what different
models were available, it’s time to apply some high-performance modifications. With the correct
modifications in the right places, the Quadrajet has no trouble being up to the task of powering your
new engine combination. Even if you are still working with a basically “stock” engine, the
modifications mentioned in this chapter help improve performance, driveability, and fuel economy.
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This custom high-performance
Quadrajet has been tested and
is ready for shipping. Note the
modified fuel-filter housing with
the -8AN fitting.
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Base Plate Modifications
The first place to start is the base plate. It is quite rare to find a base plate where the throttle plates
reach a full 90-degree angle at full throttle. It’s also quite common to find units with bent shafts and
throttle plates that don’t fully seat in the closed position.
Once you’ve cleaned the base plate and installed bronze shaft bushings, hold it up to a light to
insure that all the throttle plates are fully closed. This should be done prior to putting Loctite on the
screws and staking them so they don’t fall out. Once you’re content that the throttle plates are
tightly shut and the shafts are indexed, open the linkage to full open and check to see if the throttle
plates all reach a full-open position. A machinist’s ruler is helpful here, but we can do a pretty good
job by simply observing them. The eye picks up the difference when there is not an even distance
from both sides down to the base plate flat surface.
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It’s quite rare to find a factory
carburetor with all the throttle plates
opening to a full 90-degree angle. A
machinist’s ruler laid flat on the bottom
of the casting makes a great tool to
determine when the throttle plates are
fully open.
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The first thing to do is to create a stop for the secondary shaft if one is not present. Many early
units had a stop as part of the base plate and linkage, but later units did not. The only way to
guarantee that the secondary throttle plates always reach a full-open angle is to provide a positive
stop for them. Most models that do not have a positive stop do have secondary lock-out linkage on
the right side of the secondary shaft. Grinding a small, flat area followed by bending the short side
of the lock-out rod creates a positive stop to ensure that the throttle plates always reach a full-open
position.
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A positive stop is added to
ensure that the throttle
plates always open and
stop at exactly 90 degrees.
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Next, make sure the primary throttle plates open fully. Most roll past center. Bending the linkage
where it is contacted by the throttle speed screw acts as a stop when the linkage hits the base plate
at full throttle.
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Most primary throttle shafts
rotate far enough to have
the throttle plates well past
center at full throttle. A pair
of pliers is used here to
bend the throttle stop for a
full 90-degree opening.
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After this procedure you typically find that the linkage is not opening the secondaries all the way.
There were several styles of linkage used to apply the secondaries. Observe the contact between
the parts and gently bend as needed to get the secondaries to a full-open position just as the
primary throttle shaft linkage contacts the base plate. This procedure often takes a few minutes, but
it’s well worth the effort. We’ve observed an engine power loss during dyno testing any time the
throttle plates did not reach a full-open position.
Once you’re happy with the throttle angles at full-open and the throttle plates are fully closed at
rest, you can move on to other base-plate modifications. Remove the idle-mixture adjustment
screws and measure the size of the hole under them. Most late-model carburetors have very small
holes present here. A good starting point is to drill the holes to .090 inch. This is fine for most
applications and makes sure that it does not require turning the mixture screws out four, five, or
even six turns to get enough idle fuel to the engine. Having .090-inch holes under the mixture
screws is not always needed, especially on stock rebuilding of late-model emission carburetors, but
it doesn’t hurt anything and often provides more control of the mixtures at a much lesser setting
once the carburetor is placed in service. After drilling the holes, gently seat the mixture screws. In
most cases the springs under the screws coil bind before the screws seat. Clip about one coil from
the springs and seat them again. Continue clipping small amounts from the springs until they are
fully seated. Failure to do so causes loss of control of the idle mixture once the carburetor is placed
in service.
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Once the primary throttle
plates are adjusted for a full
open angle and a positive
stop for the secondaries is
made, bend the linkage as
needed to make sure the
secondaries reach the full
open position.
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Idle bypass air was used on many models. If idle bypass air was not used, most carburetors have all
of the passages present. Adding idle bypass air is seldom needed with near-stock engines and
very mild camshafts. The amount of bypass air to use varies greatly for different applications. For
near-stock engines with no compression-ratio increase and a slightly-larger-than stock cam, start
out with .050 to .080 inch of idle bypass air. It is recommended that the holes be drilled slightly
smaller than the desired size with a power drill, then final-sized to the desired dimension by hand
using a pin vise to hold the bit. Also make sure that holes are drilled in the main body in the
passages leading down to the bypass air holes in the base plate. Most early carburetors do not
have the main body drilled or have provisions in the base plate for bypass air. It can be added, but
precision measurements and drilling is required. For the early carburetors, it’s often just as easy to
drill small holes in the primary throttle plates if bypass air is required. Although not quite as effective
as using the bypass air system, it does help to keep the primary throttle plate angle lower at idle
with big cams. Many late-model carburetors have the idle bypass air system in place but no holes
drilled through the passages in the base plate.
Once you have completed the needed base-plate modifications, it should be fitted to the main
casting. Perform a quick test-fit without using a gasket to make sure that the epoxy you have used
to seal the bottom plugs is not holding the base plate so high that the gasket does not seal. Any
epoxy on the sides of the plugs may also affect alignment and not allow the guide pins on the main
body to locate the base plate.
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Once you have a good test fit, check the gasket for alignment on both the main body and base
plate. There were quite a few different gaskets used between the main body and base plate. Some
rebuild kits come with several styles; make sure to match up each hole between both parts.
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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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Other items you might be interested in
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Holley - Rebuilding and Modifying
This unique book takes you step-by step through disassembly, parts
inspection, modification, reassembly, installation, and on-car tuning of
all popular Holley modular carburetors. This hands-on guide includes
clearly labeled steps, hundreds of detailed photos, and easy to
understand instructions that make working on Holleys as simple as
1-2-3.
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Price:
$18.95
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How to Rebuild and Modify Carter/Edelbrock Carburetors
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/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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Shipping is combined and discounted for multiple item purchases!
Buy more and save on shipping!
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MRE
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