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How a Carburetor Works
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Complexity: Myth or Fact?
In the days of the Model T Ford, a mechanic didn’t need a great deal of expertise to be considered
a “carburetor expert.” The unsophisticated single-barrel fuel mixers used in those days had few
parts, were easy to disassemble and suffered primarily from problems induced by impurities
contained in the gasoline that they crudely metered. Today, the situation is markedly different.
Electronic fuel injection has replaced carburetion on new vehicles and with each passing year;
information and advice concerning all aspects of carburetor operation becomes increasingly more
difficult to find. At the same time, some of the more exotic carburetors of the past have taken on
legendary attributes. Four-barrel carburetors have achieved an especially undeserved reputation
for being difficult to deal with and as impossible to comprehend as a politician’s logic. And even
though numerous books and magazine articles covering the subject have been written, the myriad
of past and present carburetor designs and models has generally served only to muddy the waters
of understanding.
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Compared to an AFB, this
Carter model U-1, which was
the company’s first down-draft
model, looks primitive at best.
Modern carburetors are
certainly more sophisticated
than this 1931 model, but the
basic venturi function is very
similar.
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To the uninitiated, this is just an
ordinary Dodge Charger, but
the educated eye will
immediately detect the “HEMI”
insignia on the side. The
Chrysler Hemi did much to
perpetuate the aura of mystery
that surrounds the Carter AFB.
(Robert Genat)
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Carburetors in general, and Performer/AFB and Thunder/AVS series carbs in particular, are
reputed to be complicated. This has to do with the fact that early AFBs, having been the original-
equipment carburetor on such legendary engines as the 409 Chevrolet, Chrysler Street Hemi and
Pontiac GTO, were shrouded by the Muscle Car mystique. In truth, neither the AFB nor AVS
designs are difficult to comprehend. The same can be said of the WCFB and Thermo-Quad.
Except for a few eccentric designs, which were originated by equally eccentric engineers, all
automotive carburetors produced since the late 1950s are very similar in concept. Moreover, they
are identical in basic function; differences lie in the methods used to accomplish those functions.
However, Edelbrock and Carter carburetors—not being of a modular design—do not fully bare their
most intimate fuel circuits, even when completely disassembled. This does tend to raise the
confusion factor, but with a few cutaway drawings confusion fades like a politician’s pre-election
promises.
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Back to Basics
The basic purposes of any carburetor, be it a one-, two-, or four-barrel, are atomization of fuel,
mixing of air and atomized fuel in the amounts required by engine load, and regulation of engine
speed. You can understand these concepts most easily by spending a few minutes exploring the
theory of carburetion (the mixing of fuel with air) and then relating that theory to functional
hardware as incorporated in various carburetor models. As you might expect, you won’t have to
read much further before you bump into a few paragraphs that cover these topics.
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Air is introduced into the fuel at
two distinct points in the typical
carburetor. First, air metered
through an air bleed is drawn
into the fuel in the
emulsification tube. Then,
additional air is mixed with the
emulsified fuel in the booster
and main venturis. When fuel
reaches the intake valve, it is
almost completely vaporized.
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In order for gasoline to burn within an internal combustion engine, it must be rendered into a near
gaseous state by thoroughly mixing it with some amount of air. Air is necessary to provide oxygen, a
requisite of combustion. During idle and part-throttle operation (such as steady-state cruising and
light acceleration) 14.7:1 is the chemically ideal or stoichiometric air/fuel ratio. Under full-throttle
and heavy-load operation, a richer mixture (in the vicinity of 12:1 or 13:1) is required to produce
maximum horsepower and keep combustion chamber temperatures (and spark knock) under
control. (During the 1970s and ’80s, some automakers experimented with “lean burn” technology as
a means of increasing fuel economy. Unfortunately, air/fuel ratios leaner than 15:1 have a markedly
negative impact on exhaust emissions.)
Suction, caused by the pistons moving down their bores during the intake cycle, creates a partial
vacuum in the induction system. In turn, these low-pressure pulses are communicated through the
intake manifold to the carburetor discharge nozzle and cause fuel to flow into the air stream. At this
point, the fuel—pouring into the fast-moving air—is atomized (converted to a spray of fine droplets).
It has not necessarily been vaporized. It remains in small droplets until it reaches the low-pressure
(high-vacuum) area, beneath the carburetor throttle plates, that enables it to “boil” and become a
gas. Strong manifold vacuum is a necessity not only for good atomization, but also to provide a
proper (low pressure) environment for vaporization—essential for both peak efficiency and
maximum performance.
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The front AFB on this factory Hemi
installation has no provision for idle-
mixture adjustment. Under this
arrangement, the rear carb is
considered a “primary”—with an idle
circuit—while the front carb serves as
a “secondary.” Since the front barrels
of the rear carburetor are centrally
located, fuel distribution is
acceptable.
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Efficient combustion of gasoline is only
possible when the fuel is mixed with 13 to 17
times as much air (by weight). The mixing,
performed remarkably well by the relatively
simple automotive carburetor, accurately and
uniformly meters small quantities of fuel into a
rapidly moving air stream.
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When fuel droplets, in this case from the idle
slot, enter the low-pressure (high-vacuum)
area below the throttle plate, they “boil” into a
near complete vapor and thoroughly mix with
the surrounding air.
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As the piston moves down the
cylinder during the intake
stroke, a lower pressure is
created in the combustion
chamber, intake port, and
manifold runners. Air from the
outside rushes in the
carburetor to equalize this
pressure differential, drawing
the air/fuel mix into the cylinder.
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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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