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Carburetor Modifications
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Why Modify?
There appears to be a great disparity in the type of carburetor modifications performed by racers
and performance enthusiasts; there is also an equal disparity in the degree of success achieved
through those modifications. It would seem that Murphy’s Laws (anything that can go wrong will go
wrong, just because you’re paranoid doesn’t mean they aren’t after you and countless others) are
invoked whenever a carburetor is disassembled. And more often than not, simple misunderstanding
is the culprit. You must comprehend the “whys” of a modification along with the “hows” if you are to
experience the thrill of victory, rather than the agony of defeat.
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Carter and Edelbrock
carburetors “need” few
modifications to provide
consistent performance. Well
established in the Pontiac ranks,
but widely used on almost all
high-performance engines, the
simple, straightforward design of
these carbs permits easy tuning,
modification, and repair. (Robert
Genat)
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For openers, the original carburetor application dictates that calibrations fall within certain pre-
determined parameters. And since each carburetor circuit interacts with others, a simple
modification can cause untoward ramifications, with the end result being a carburetor that suffers
from “terminal dysfunction.” Whether you’re attempting to modify a carburetor to achieve improved
performance, better drivability, or greater fuel economy, it is essential that you understand original
design philosophies if you expect to achieve the desired results. Also note that radical alterations
should never be necessary for anything other than highly specialized racing applications. The need
for extensive rework is usually indicative of a damaged carburetor or misapplication.
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Tuning and modification should
be done from a “known” starting
point. It’s wise to check original
specs before getting started.
This can be done by locating
the model number. Some carbs
have the number on identifying
tags.
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Others carbs have model
numbers stamped in a boss on
the base plate. But wherever
you locate it, jot it down for
future reference; it’s nice to
know that you can “undo”
modifications that don’t yield
the desired result by ordering
stock replacement parts.
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Primary Idle System
Most carburetors, including the AFB and AVS, meter idle fuel through the same type of circuit—one
that incorporates adjusting screws with needles that protrude into the curb idle discharge port.
These screws are typically rotated clockwise to lean the idle mixture and counter-clockwise to
richen it. However, some AFB and AVS carbs were produced with a single mixture adjustment screw
controlling the flow through both primary barrels. Under this arrangement, bleed air, rather than
air/fuel emulsion, is adjusted when the screw is rotated, but mixture is varied according to the same
procedure used with conventional idle systems. When a single adjustment screw is used, it is left-
hand threaded so that rotating it clockwise leans the mixture. This occurs because clockwise
rotation of a left-handed screw causes it to back out, or loosen. Carter carburetors fitted with air
bleed idle mixture adjustment also have fuel emulsion mixture needles, but these are sealed at the
factory to limit the amount of fuel admitted during idle. Inspiration for this arrangement was provided
by the federal government through its exhaust emissions regulations. Some original equipment
carburetors were restricted even further, with all adjustments sealed.
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Modifications
In general, there should be no need for modification of the idle system other than adjustment of
mixture and engine speed. However, if after a full range of adjustments are made and idle mixture is
too rich or too lean, either the idle air bleed or the idle feed restriction has to be modified. (This isn’t
meant to imply that these modifications must be attempted, but that they are the only options
available other than replacing the carb.) Since the idle feed restrictions (also called low-speed or
idle jets) on Carter and Edelbrock carburetors are usually placed in the main well, they are all but
inaccessible and all idle calibrations must be made by altering the complementary air bleed
diameters. Determination of air bleed size is based on the diameter of the curb idle port, desired
throttle plate position at idle, and transfer circuitry. Since all of these factors affect system timing,
alteration of bleed diameter is not as straightforward as changing fuel jets. Consequently, air bleed
modifications are generally ill advised. If you’re not in the habit of heeding advice, ill or otherwise,
keep in mind that drilling a larger hole is infinitely easier than drilling a smaller one. Another piece
of advice: If a carburetor does not provide proper or even reasonable idle characteristics with
factory calibrations, it is probably defective or is being used in an improper application.
The apparent need for idle circuit recalibration arises most frequently when a carburetor originally
intended for racing usage is installed on a street-driven vehicle. Race carburetors are calibrated to
provide a rich idle mixture as a means of compensating for exhaust dilution of the intake charge (a
result of high cam overlap), and the relatively low fuel-metering signal that results from extremely
long camshaft duration. When installed on a meek, mild-mannered street engine, a carburetor with
race calibrations may feed excessive amounts of fuel at low speeds. This not only affects drivability
but seriously degrades fuel economy, hence the need to lean the mixture. However, while
enlargement of air-bleed diameter provides a leaner emulsion, this change also impacts circuit
timing (the point at which idle fuel flows), and if too much of the signal is bled off, the idle circuit may
stop delivering fuel before the main circuit begins. This can result in an off-idle stumble that causes
the car to break into an automotive version of the foxtrot.
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If you are prepared for the consequences and there is no alternative but to drill the bleeds larger,
measure the existing diameter before you begin. If too much material is drilled away, epoxy can be
used to seal the air bleed so that it can subsequently be redrilled to the original diameter—or to the
size that seemed to work so well when you decided to “give it just a little more to be sure.”
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When adjusting the idle air/fuel mixture on
most carbs, the adjusting screws are turned
clockwise to lean the mixture and counter-
clockwise to richen it. However, most emissions-
type carbs employ a “reverse idle” system in
which adjusting screws have to be turned
counter-clockwise to lean the mixture.
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In addition to throttle plates, the lower portion
of a carburetor contains numerous air
passages. Idle problems can result if these
passages become obstructed or the base
gasket used with the carb allows air leaks.
Keep everything clean and use high-quality
gaskets.
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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
Carter/Edelbrock Carburetors
by Dave Emanuel
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If you want to modify your carburetor-equipped vehicle for
performance today, you can choose from three major carburetor
families – Holley, Demon, and Edelbrock. While Holley and Holley-
style carburetors like Demon have been popular for years with
their well documented successes and quirks, there has never
before been a book covering the ins and outs of the emerging
Edelbrock line of carburetors.
How to Rebuild and Modify Carter/Edelbrock Carburetors reflects
the emergence of Edelbrock carburetors as the predominant
Carter-style carburetors in the market today. A revision of the
best-selling title Super Tuning and Modifying Carter Carburetors,
this book contains more than 300 color photos, illustrations, and
diagrams, covering rebuilding, tuning, and modifying Carter and
Edelbrock carburetors.
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Click below to view
sample pages!
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1 - History of Carburetors
2 - Carburetor Types
3 - Selecting a Carburetor
4 - How a Carburetor Works
5 - Carburetor Modifications
6 - Carburetor Tuning
7 - Rebuilding Tips
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This book also features the history of Carter as well as the history
of the AFB and the AVS since the purchase by Edelbrock. 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.
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This is a great book
that anyone who
owns or works on
Carter or Edelbrock
Carburetors will love!
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8-1/2 x 11"
Softbound
136 pages
159 color and 156 b/w photos
Item # SA130
Price: $22.95
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Click here to buy now!
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Rebuild and Modify Quadrajet Carburetors
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. This is the only book to truly cover
step-by-step instruction on performance building the
Rochester Quadrajet!
A complete guide to selecting,
rebuilding, and modifying the Q- Jet, aimed at both muscle
car restorers and racers.
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Price:
$
22.95
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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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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
purchases as follows: first item regular shipping price, add
$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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