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CARBURETORS!
Barry Grant Answers the Most Frequently Asked
Questions
By Sam Moore
Carburetor guru, Barry Grant, a man who has done more than most to
exploit the boundaries of carburetion.
Given
that a four-barrel carburetor possesses something in the order of 200
components to allow it to function correctly, it’s no wonder the average
enthusiast has the odd question about how it operates and why.
Words and phrases like atomization, vacuum signal, fuel-metering
circuits, etc. can be a little bewildering.
So, here are a few of the basics with several useful illustrations
to help explain. This article
contains information that applies to a wide range of V8-engined machines,
from street cars to tow vehicles, and from oval track racers to drag strip
cars.
1)
Q. Is air pushed or sucked into a carburetor?
A.
With the exception of forced induction systems (centrifugal
supercharger, or turbocharger - where air is forced through the
carburetor), air is drawn through the carburetor (sucked) by the
engine’s vacuum signal. The
amount of air is determined by the strength of the vacuum signal.
2)
Q. What is the difference between the straight-leg venturi booster,
the down-leg booster, and the annular-discharge booster?
Also, when should one be used in preference to another, and how
does it affect jetting?
A.
A straight-leg booster has, as its name implies, a straight leg,
which protrudes from the body of the carburetor into the main venturi. Its
discharge ring is situated slightly above the venturi’s most effective
zone.
B.
A down-leg or drop-leg venturi booster drops the discharge ring
lower in the carburetor’s main venturi where it operates in air of
higher velocity, which draws more fuel than the straight-leg style of
booster.
C.
The annular-discharge venturi booster has a larger ring with
multiple discharge holes rather than the single outlet hole of the
straight-leg or down-leg style of booster.
This has the effect of creating a venturi within the main venturi
that produces greater vacuum than either the straight- or down-leg
varieties and draws even more fuel.
D.
In conclusion, the pros and cons of each type of booster are based
largely upon its application. However, if each style is tested in the same carburetor with
all else being equal, the down-leg booster will require smaller jets to
flow the same amount of fuel than a straight-leg booster. And the annular-discharge booster will require even smaller
jets to flow the same amount of fuel as the down-leg type.
3)
Q. Mechanical secondary or vacuum secondary, which style of
carburetor should I use and why?
A.
Generally speaking, a mechanical secondary carburetor is preferred
on vehicles with manual transmissions and on automatic transmissions with
3000+ RPM stall-speed converters. For automatics with less than 3000-RPM stall-speed
converters, the vacuum-secondary carburetor is usually the better choice.
4)
Q. How do I set the float levels and what effect do they have?
A.
On Demon carburetors in street-driven applications, begin by
setting the float levels at ¼ distance of the sight window.
On race applications, increase the levels by setting them between
½ and ¾ distance in the sight window. On Holley carburetors, remove the
level plug and adjust the float level until fuel trickles from the open
port. Changing the float levels alters the amount of fuel in the bowl
(reservoir) and the carburetor’s ability to feed the main jets.
By raising the float level the engine’s response is quickened.
This is an adjustment frequently used to eradicate a lean stumble.
By lowering the float levels, the activation of the main metering
circuit is delayed and consequently produces a leaner mixture coming off
idle. For street applications
the latter condition is more economical.
5)
Q. Does a bigger carb make more power? What’s the limit?
A.
A larger carburetor can produce more power on a dynamometer, but
under normal operating conditions can result in slower acceleration and
lower efficiency of fuel atomization.
Select the smaller carburetor, especially if you’re undecided
about sizes. The smaller
diameter venturii increase the velocity of the air/fuel mixture.
As a consequence, it usually provides better acceleration and
proves to be more efficient.
6)
Q. Where should the fuel-pressure regulator be located and what
type should I use?
A.
The pressure regulator should be mounted close to the object
(carburetor, nitrous system, etc.) that’s being fed with fuel.
The further away the regulator is placed the greater the delay in
its response (its opening and closing). A slower response causes
fluctuations (spikes) in the fuel pressure. Regarding the selection of a regulator, specific vehicle
requirements, but especially the type of fuel pump already in use dictates
the type of fuel-pressure regulator required.
For example, combine a block-mounted 15-psi pump with a throttle
bypass; a belt-driven pump with a diaphragm bypass; a BG280 electric pump
with a two-port regulator and a BG400 with a four-port regulator.
7)
Q. How does weather and altitude affect carb jetting?
A.
The more oxygen there is in the air the more fuel the engine will
demand. For example, as the
weather becomes colder (winter) or the altitude lower (closer to sea
level), the air will contain more oxygen and the engine will require a
larger jet size. In contrast, as the weather becomes warmer (summer) or
the altitude higher (mountainous) the jet size needs to be reduced.
Images
and Captions
All photographs courtesy of Barry Grant, Inc.
Atomization test.jpg (relates to Question #1)
To
understand the carburetor and its function it’s helpful to have an image
in mind of atomization, it’s the critical factor in carburetion.
Maintaining high velocity at the point where the fuel mixture is
introduced to the air stream is key to achieving optimum atomization of
the fuel.
Third Image: straightlegbooster.jpg (relates to Question #2)
The straight-leg venturi
booster protrudes from the body of the carburetor into the main venturi.
Its discharge ring is slightly above the venturi’s most effective
zone. The color red indicates
the main fuel circuits, which connect with the main jets, the emulsion
holes (blue) and the high-speed air bleeds located in the air entry on top
of carburetor. The color
yellow indicates the idle-fuel circuits, which connect with main-fuel
wells, the idle-feed restrictors, the idle-air bleeds (also located in air
entry on top of carb), and the idle-discharge ports and transfer slots in
the baseplate.
Fourth Image: downlegbooster.jpg (relates to Question #2)
The down-leg venturi booster
lowers the position of the discharge ring in the main venturi and places
it in an area of higher air velocity, which draws more fuel than the
straight-leg type.
Fifth Image: annularbooster.jpg (relates to Question #2)
The
annular discharge venturi booster (shown in the left venturi of the
accompanying illustration) has a larger ring with multiply discharge holes
rather than the single outlet of the straight- or down-leg booster.
Of the three types of venturi boosters, the annular style provides
the greater vacuum draw
Sixth Image: Race Demon.jpg (relates to Question #3)
The
Race Demon with mechanically operated secondary throttle plates is
designed for smooth driveability on dirt or asphalt, and has customized
fuel-metering circuits developed for rapid throttle response and strong
mid-range and top-end power.
Seventh Image: Floatleveladjustment.jpg (relates to Question #4)
Set
the float levels by adjusting the internal floats with a screwdriver and
wrench and viewing the levels through the sight glasses. Perform this
operation with the engine running.
Eighth Image: VenturiSleeves.jpg (relates to Question #5)
For
versatility, use a Demon RS when one carburetor is needed for different
applications or changing conditions, i.e. when the engine size changes, or
the type of fuel changes, or the track changes or the weather changes.
The venturi sleeves pictured alter the Demon RS (removable sleeves)
from 675 cfm to 1050
Ninth Image: Beltdrive&fuellog.jpg (relates to Question #6)
The
fuel pressure regulator or bypass should be located as closely as possible
to the carburetor. Here,
pictured with its mating belt-drive fuel pump, the bypass is integrated in
the fuel log.
Tenth Image: JetPlate.jpg (relates to Question #7)
Eleventh Image:
Service Pack195003.jpg (relates to Question #7)
Use
a jet plate kit to protect and to store spare jets.
Also, keep a carburetor rebuild kit in the toolbox.
They’re inexpensive and good ones (like those from Barry Grant)
are supplied with every gasket and seal necessary, they even contain a
power valve and detailed instructions.
copyright 2004 Stedman Marketing Group
All rights reserved. Unauthorized use without permission is prohibited.
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