The
part of an engine which mixes fuel and air in the right ratios,
producing the gas which is burnt to provide the power needed to
operate the vehicle.
Carburetion
refers to the use of a carburetor as a means of controlling an
engine's air/fuel ratio. Carburetors were used on most cars through
the mid-1980s, when carmakers began a large-scale changeover to fuel
injection. A carburetor holds fuel in a small reservoir called a float
bowl. This reservoir is connected to
a passageway leading to a venturi,
a device that uses pressure differential to help meter fuel into the
engine. Conventionally referred to as "barrels", it refers
to the number of venturis in the carburetor. A one-barrel carburetor
has one venturi; a two-barrel carburetor has two venturis, and so on,
up to four venturis. Around 1980, carmakers began to add mixture
control solenoids and other electronic devices to carburetors, to
make them more effective by allowing additional control through an
electronic engine control system.
A
float carburetor, in which a float
regulates the fuel level in a reservoir from which the fuel is sucked
into the intake manifold at a restriction called a venturi.
This venturi metering system controls the flow of a continuous
pumped spray into the intake manifold downstream from the carburetor.
When
there is an individual spray for each cylinder and the injection is
an intermittent, timed spurt, or is metered differently, the device
is usually called a fuel
injector, NOT a carburetor.
One of the most
misunderstood parts of the engine, the carburetor is essentially a
basic air/fuel meter that regulates how efficient that giant air pump (your
engine) under the hood is.
A carburetor does not have
an easy job. It must add fuel to the incoming air and mix it
thoroughly for efficient combustion. It must do this at any engine
speed, load or throttle setting.
It is also the
carburetors job to regulate the airflow into the engine to control
engine speed.
All carbs work on the same
principal, pressure differential. Without differences in pressure, a
carburetor just will not work. So where do the differences in
pressure come from? the first is from the engine pumping air out of
the intake manifold. This creates a lower than atmospheric pressure,
then atmospheric pressure will push air in. The second is from the
venturi effect. As air flows through the narrow part of the venturi,
the velocity increases and the pressure will drop. If the pressure
drops lower than fuel pressure, which is at atmospheric, atmospheric
pressure will push fuel into the venturi.
Technically
Speaking
Because the current slew of
aftermarket four-barrels available on the performance scene today
benefit from cutting-edge design and engineering, and to truly take
advantage of the power and reliability these units offer,
one must first understand
how the carburetor works and how to properly adjust them.
Hopefully after
reading this article you'll be well versed in fuel metering lingo.
Whether you're running a
fully tricked-out modern four-barrel
or
a box-stock Model A single-pot,
all carburetors work
because of a theory called
The Bernoulli Principle,
which explains not
only how fuel delivery systems work but the basics of lift and
flight, as well.
The principle is this:
As the velocity of a
gas increases, the pressure drops. This change in pressure is linear
to the change in velocity. When a piston
in your engine drops down in the cylinder
on the intake stroke, it creates a vacuum in the cylinder
compared to the atmospheric pressure of the outside world. Pressure
is constantly trying to regulate itself, so air comes rushing from
the outside (in this case, your engine compartment), through
the carburetor, and into the empty cylinder.
As the air is pulled into
the carburetor venturi, a
tube with a constriction; used to control fluid flow (as in the air
inlet of a carburetor) it has to accelerate from a
standstill, and the acceleration is regulated by the speed of your
engine and the position of the throttle blades in the carburetor. The
pressure difference between the carb venturi with air rushing through
it and the higher atmospheric pressure of the outside world creates a
vacuum, which actually pulls fuel out of a reservoir in the
carburetor (called a
float bowl), through a small port (called
a jet), and into the airstream. As stated in the
Bernoulli Principle, the faster the air moves, the higher the vacuum
gets, which means more fuel is pulled into the airstream; therefore,
no matter what the engine speed, the fuel-to-air ratio stays
constant. If you have the proper-size venturis that will flow the
necessary air for your engine, and if the jets
supply the right amount of fuel, a carburetor will supply your engine
with the perfect amount of fuel (gas and oxygen) under just
about any condition.
Consequently, factory
automobiles came equipped with them for more than 80 years.
This also explains why you
can take a tiny two-barrel carb and bolt it to a healthy big-block,
or take a giant four-barrel race unit and stick it on a six-cylinder,
and both engines will run. Granted they won't make optimal power, but
because the air/fuel ratio is always naturally balanced, both engines
will have enough properly balanced fuel and oxygen to run as long as
the jetting is correct.
CHOOSING
A CARBURETOR
One thing that has changed
over the years is the selection available to the modern consumer. As
we mentioned earlier, there are several companies currently
developing and manufacturing a wide variety of carbs for the
performance market, so the big trick is finding the perfect piece for
your ride. Although just about any carb will work on any motor, it
doesn't necessarily mean it will work well.
Wen it comes to fuel
delivery, bigger is not necessarily better. A motor with too much
carb will not want to idle, can run weak off idle, and will be
sluggish all around. Then again, too small a carburetor will limit
the horsepower your engine can produce, and since decent carburetors
don't come cheap, you don't want to end up playing a trial-and-error
game with your fuel delivery system. The best way to go is to first determine
the cfm(cubic feet per minute of airflow)
necessary to keep your mill well fed, then dial-in exactly what type
of unit will perfectly suit your car and driving style.
The next step after
determining the cfm of your carburetor is to choose whether you need
vacuum or mechanical
secondaries. The beauty of a four-barrel carburetor is
that most of the time the engine only needs a very small amount of
fuel to run, so the first set of venturis,
called primaries, are in action. When necessary under heavy
acceleration, the second set of venturis, called secondaries,
open up and provide what is essentially an extra set of lungs. The
most common type of carburetor uses vacuum-operated secondaries,
which utilize and increase in load and engine vacuum to gradually
pull open the extra throttle blades. However, in engines with
extremely large-duration camshafts where low-speed vacuum (or lack
thereof) is an issue, this is not always the ideal setup. The other
form available is a mechanical secondary, which allows the driver to
control the secondary system. While control sounds like a great
thing, in many situations if a person slams his paw down to the
pedal, the resulting dump of air down the motor can actually cause a
huge lean spot as vacuum fails to pull in enough fuel for a proper
ratio, causing a flat spot in acceleration. This is where vacuum
secondaries come in handy.
Relatively mild motors with
stock- or RV-grind camshafts and rods that are mostly used for
cruising are usually better off with a vacuum secondary carburetor.
For detailed information on how vacuum secondaries function see the
illustration in this article, but in a nutshell this type of system
utilizes increased engine vacuum and load at high engine speeds or
under acceleration to pull open the second set of throttle blades and
thereby gradually increase the amount of air and fuel going into the
motor as necessary.
Mechanical
secondaries come in handy with large motors built to make
serious horsepower, as long as the power
valve, jet, and ramp settings are correct. Rather than
relying on vacuum pulling a diaphragm, mechanical secondary carbs,
which are often also "double pumpers" with two accelerator
pumps, utilize a mechanical linkage to open the second
set of throttle blades. These units are best in high-performance
motors with lots of cam and very little vacuum, as well as in certain
heavy-duty applications where a large vehicle might be pulling too
much vacuum under loads, thereby using secondaries unnecessarily and
decreasing fuel mileage. In this instance the control of a mechanical
secondary is preferable.
Finally, once you have
chosen the size and style of carb needed for your application, it's
time to choose a brand. Holley, Demon, and Edelbrock all make
excellent products in a wide array of sizes and styles designed to
suit a variety of applications. All three manufacturers have tech
lines open where professionals are available to help you make a more
informed decision, and there are several aftermarket tuning shops
that also specialize in taking things a step further by
custom-building carburetors for a customer's unique, individual application.
Now follow along as we
detail the various parts and pieces that make the typical carburetor work.
BITS
AND PIECES
Choke
Before an engine reaches
operating temperature, the incoming air will not be heated as much as
it would if the engine was at operating temperature. The result is
denser intake air, which contains more oxygen. A carburetor does not
have the ability to sense air density, so it cannot add fuel to
match. There must be an air valve above the venturi
to limit incoming air until the engine warms up. The choke valve also
increase vacuum in the venturi, which richens the mixture. The choke
also incorporates a fast idle, which will help fuel atomization when
cold and help the engine warm up faster. The choke valve,which can be
either manual or electrically operated, will open slowly as the
engine warms up and the fast idle should come down to the regular
idle in a series of steps. Not all fast idles are automatic,
sometimes you must tap the gas pedal to allow the cam to move.
Float
The fuel level in the float
bowl is very important to carburetor response and
consistency, so there needs to be something to keep the fuel at the
proper level, this is where the float
comes in. The fuel pump fills the float bowl and as long as it can
pump sufficient fuel for peak fuel demand, there should be no problem
filling the float bowl. The floats job is to reduce fuel flow when
fuel demand is low. The float simply floats in the fuel and pushes a
valve closed as it rises. The valve is known as the needle valve. As
the fuel pump fills the float bowl faster than the engine is using
fuel, the float will rise and close the needle valve more to reduce
fuel flow. There must also be enough float drop to allow the needle
valve to open fully when fuel demand is high.
Float Bowl
All circuits in a
carburetor get fuel from the float bowl. The float bowl is just a
fuel reservoir where all the circuits in a carburetor get fuel. The
float bowl is also sometimes called a fuel bowl.
Float Bowl Vent
This is what vents the float
bowl to atmospheric pressure, the pressure that pushes
fuel through the passages when the pressure at the other end of that
fuel passage is lower than atmospheric. Pressure differential is what
makes any carburetor work. In the case of forced induction, the bowl
vent must be pressurized the same as boost pressure. If venturi
pressure is greater than float bowl pressure, no fuel will get into
the engine.
Intermediate Circuit
Some large carburetors used
an intermediate circuit to provide fuel after the idle
circuit, but before the main system. Most Holley
Dominator's have an intermediate circuit because the venturi
are large and need more airflow for the main system to start working.
The intermediate circuit
fill the gap.
Idle Feed Restrictions
The curb idle and transfer
slot fuel comes from the idle well and is controlled by the idle feed
restrictions and idle air
bleed. The curb idle will also have an adjustment screw
to fine tune the curb idle, but the transfer slots will have a fixed
flow that can only be changed by changing the size of the idle feed
restrictions or idle air bleeds.
If these circuits
need adjusting, the carb is most likely mismatched for the application.
Accelerator Pump
The
accelerator pump supplies fuel pressure to compensate for losses in
fuel flow when the airflow signal to the booster venturis
diminishes as you accelerate off idle.
When the throttle is opened
quickly, there is a delay before the main
metering system can flow enough fuel. The idle
circuit, which flows fuel through the curb idle discharge
ports and the idle transfer slots, will stop flowing fuel quickly as
vacuum drops. If you look at the small size of the idle
feed restrictions and the large size of the idle air
bleeds, you can see that it takes rather high vacuum to
make the idle circuit work. To cover the fuel gap, there must be an
acceleration circuit to fill the gap. The accelerator pump simply
squirts a timed shot of fuel to fill that gap. The orifice that the
fuel exits the pump is called an accelerator pump squirter or a
shooter, the size affect the volume and duration of the pump.
Jets
Jets are restrictions in a
circuit to control fuel flow through it. Most carbs will have
replaceable jets to make for easy changes in calibration
The jets are small,
threaded plugs in the base of the carburetor in the metering block
which regulate the amount of fuel that flows from the float
bowl into the venturi.
The ideal carburetor will have a perfect air/fuel ratio, which can
sometimes mean a few jet adjustments. The easiest way to check your
jetting without an air/fuel meter is to find a nice stretch of
deserted road, accelerate hard, then shut the motor off before it has
a chance to idle. Then pull a few spark plugs and check the color of
the porcelain. It should be a nice light-brown-cocoa color. If it's
any lighter, your motor is running lean, which could hurt power and,
in a worst-case scenario, even burn a piston. If the motor is running
rich, the color could be dark brown or even black. A good way to
adjust jet size is to go up or down as necessary two sizes at a time
until the optimal ratio is set.
Jet Extensions
The rear jets
of a Holley or Demon type carburetor can be uncovered on hard
acceleration. By extending the jets to pick up fuel toward the center
of the float bowl,
it makes it harder to uncover the jets.
Air Bleed
If fuel went directly to
the venturi
from the float bowl,
it would discharge as large droplets. Since a good air / fuel
mixture is important to efficient combustion, large droplets would be
a poor way to start things off. The air bleeds allow air to enter the
circuits. Through emulsion
tubes, the air is mixed with the fuel. So an air / fuel
emulsion is what is discharged in the engine, not just fuel. Another
function of an air bleed is to prevent a siphoning effect form
draining the float bowl into the engine when it is shut off. I'll
talk more about the importance of air bleeds in the Carb Basics article.
Air Valve Secondaries
Many factory carbs use an
air valve to open the secondaries.
There are the usual throttle plates that open mechanically, but a
second set of plates is opened based on air demand. The mechanical
throttle plates assure than the secondaries are closed when engine
braking. To put it simply, air pulls the secondaries open. There must
be some form of resistance to control the opening rate. Rochester
Quadrajet's and Holley Economizers use a simple spring, the stiffer
the spring, the slower they will open. Carter AFB's and Edelbrock's
use counterweights, the heavier the weight is, the slower they open.
Since they open with air demand, they will not open fully if the
engine is not pulling in enough air, so an overly large carburetor
can work just fine.
CFM
Cubic Feet per Minute of
airflow. Most 4 barrel carbs flow ratings are taken at 1.5 inches of
vacuum and 2 barrels are measured at 3 inches.
HOW TO CALCULATE CFM
Multiply the cubic inch displacement of
your motor by the maximum rpm the engine will attain, then divide the
total by 3,456. The resultant number will be the cfm necessary for
your engine to run at 100 percent volumetric efficiency. For you math
guys, the formula looks like this:
The engine in the above example would need
a 600-cfm carburetor to run at 100 percent volumetric efficiency.
Only the mostly highly modified and efficient race engines can get
even close to 100 percent efficiency. Most street motors are closer
to 85 percent, but the number is still good for a baseline.
Therefore, a 600 carb would be just about perfect.
Emulsion Tubes
The fuel that is discharged
from a carburetor is premixed with some air to make an air fuel
emulsion. This is done in the emulsion tubes and the air comes from
the sir bleeds. The air
bleeds connect to the emulsion tubes in the well. The
tubes extend down into the fuel well and have small holes to allow
air to get pushed in to the fuel as the fuel travels up fuel the
well. The amount, size and location of the holes in the emulsion
tubes all have an effect of how the air mixes with the fuel.
Hesitation
When there is a lean or an
extremely rich spot in the fuel delivery curve, the engine can
hesitate. There are many deferent opinions of this but in general, if
the hesitation is caused from a rich condition, some call that a bog.
If it is causes from a lean condition, some call it a stumble.
Main Metering System
Main system fuel is
controlled by the main jetting, the power
valve circuit and the high-speed air
bleeds. The fuel from the main system is discharged into
the venturi,
usually out of the center of the booster venturi, but not all carbs
have booster venturi.
Basic Main Metering System
As we stated before,
airflow through a venturi
will cause a pressure drop at the narrow point. This will introduce a
lower than atmospheric pressure at the discharge nozzle. A booster
venturi can be used to further decrease pressure and help
fuel distribution and atomization. The fuel is at atmospheric
pressure due to the bowl vent.
With the pressure at the
discharge nozzle being lower than the fuel, the fuel will be pushed
out of the discharge nozzle. the amount of fuel is controlled by the
size of the hole in the main jet. Larger jets will flow more fuel and
result in a richer mixture.
If we left it at that, the
fuel will exit the discharge nozzle in large droplets, and since we
want a fine mist, it's not a good thing to do. The air bleed is teed
in to the main well and since it too is at atmospheric pressure, air
is pushed in at that point as also. How much air depends on how large
the air bleed is. the larger the air bleed , the more air will enter
and the leaner the mixture will be. The air must travel through the
emulsion tube, which has small holes in it to mix the air more evenly
in the fuel. As the fuel travels up the main well, air is added to it
through the emulsion tube, so as it exits the discharge nozzles ii is
an air / fuel emulsion. This make much finer fuel droplets that are
easier to mix with the incoming air. An air bleed does kill some of
the vacuum signal at the main jet and delay the activation of the
circuit. Altering air bleed size can lead to stumble and poor fuel
atomization. Carburetor manufacturers do a lot of research to
determine the best air bleed size, and unless you know more than
them, leave them alone.
If they need to be altered,
the problem is more likely do to a mismatched carburetor.
As the air / fuel emulsion
exits the discharge nozzle, the lower pressure in the venturi will
help further break up the fuel droplets. The booster venturi will
help distribute this mixture evenly to the incoming air.
Idle Circuit
The idle circuit controls
idle and off idle fuel delivery.
Up to this point you should
have a basic understanding of how the main
metering system works, but the main system will only work
if there is sufficient airflow through the venturi
to create a large enough pressure differential for it to work properly.
When the engine is idling,
or running at very low speeds, the idle circuit will need to provide
the fuel that the main metering system cannot. The idle circuit will
control the idle fuel and the transition to the main metering system,
and mixture screws let you adjust the air/fuel mixture of this circuit..
The most popular type of
idle circuit is show in a simplified diagram below.
In this type of idle
circuit, the fuel is drawn from the main well. It passes through the
idle feed restriction, then is mixed with bleed air from them idle
air bleed. Not shown in the drawing is the idle emulsion tubes, but
they function just like the ones in the main system. This air / fuel
emulsion then travels to the curb idle discharge port. The amount of
air / fuel emulsion that exits the port is determined by the curb
idle adjustment screw. The air / fuel ratio is determined by the idle
feed restrictions and the idle air bleeds, not the adjustment screw.
The screw only adjust the
amount of air / fuel, NOT
the air / fuel ratio.
As the throttle opens, the
transition slots will become more and more uncovered to manifold
vacuum. As this happens, the air / fuel emulsion will begin to flow
from the transition slot. The curb idle adjustment screw does not
control the amount emulsion from the slot. The only thing that
controls the air / fuel ratio or amount of air / fuel, is the idle
feed restrictions and the idle air bleeds. This circuit is to make a
smooth transition to the main metering system. As throttle opens and
manifold vacuum decreases, the idle fuel will taper off until there
is not enough vacuum for the idle circuit to work.
Mechanical Secondaries
Mechanical linkages will
usually be progressive. In some cases, like individual runner
manifolds, a 1:1 linkage will be needed. Opening the secondaries
mechanically causes the same fuel delivery gap in the secondaries as
the primaries, making an accelerator
pump needed. It is possible in some cases to have a long
duration primary pump shot that covers the secondary opening, but
individual runner set ups will always need a secondary accelerator
pump. The Holley 4150 carbs are a good example of a mechanical
secondary carb that uses a secondary accelerator pump.
Metering Plate
A metering plate takes the
place of a metering block in a Holley 4160 and some 4150 carburetors.
There are no replaceable jets
in a metering plate, just drilled restrictions. Holley uses them
because they are cheaper and easier to make than a metering block.
Power Enrichment Circuit
The circuit that adds fuel
on hard acceleration and/or heavy loads.
See power
valve for more information.
Power System and Power Valve
These are
vacuum-operated valves that open and close at preset amounts of
manifold vacuum, which is measured from 2.5 to 10.5 inches. The lower
the number, the later the valve opens. These same numbers are punched
onto replacement power valves, and metering comes from small holes
directly beneath the valve. Edelbrock and Carter carbs do not use a
power valve. Instead, they rely on metering rods that run through the
main jets. As
manifold vacuum drops under power, a spring under the rod holder
raises the tapered rod out of the jet and allows more fuel to flow
past, meaning the mixture gets richer. One benefit of the Edelbrock
system is that metering rods do not blow out when an engine
backfires, which can occasionally happen with power valves.
Power Valve
A power valve adds
enrichment fuel for hard acceleration. There are many ways this can
be done. Most popular carbs use either metering rods that restrict
the main jets
when cruising and step to a smaller diameter restriction on hard
throttle (Carter, Edelbrock and Rochester use these on many carbs) or
a vacuum diaphragm valve that opens another set of fuel passages that
add to the main jetting (Holley use Demon use this method). In either
case, the valve is controlled by engine vacuum and opens when vacuum
drops to a set level. The amount of enrichment fuel is determined by
the diameter of the metering rods or the size of the power valve
channel restrictions for Holley and Demon carbs.
Power Valve Channel Restrictions
Most Holley performance and
Demon carburetors use a diaphragm power
valve the opens on hard throttle to add fuel to the main
circuit. The fuel that comes from the power valve is controlled by
restrictions called power valve channel restrictions. The larger the
restriction, the more enrichment fuel there will be when the power
valve opens. Main jet changes will change fuel flow at cruising
speeds and hard throttle, but power valve channel restrictions only
affect fuel flow at heavy and WOT conditions.
Reverse Idle System
Unlike the basic idle
adjustment screws that control the amount of air/fuel emulsion to the
curb idle discharge port, the reverse idle system controls the amount
of bleed air that enters the idle
circuit. It gets it's name because the more you restrict
the bleed air, the richer the mixture is, so turning the screw in
will richen the circuit, rather than lean it out. This system was
primarily used on low emissions engines.
Secondaries
In a multi venturi
carburetor with a progressive linkage, the secondaries are the second
set of barrels that open. In multiple carb set ups, there can even be
secondary carburetors if a whole carb is not used until a certain
amount of throttle. There are also progressive 2 barrels. The
secondaries are usually the rear set of barrels on a 4-barrel.
Secondaries also will not have a choke
on them because they are not used until hard throttle.
Throttle Body
Or throttle valve to better
describe it. This is what controls the amount of air allowed to enter
the engine. It is located on the engine side of the venturi
to limit airflow through the venturi. The throttle valve is usually a
set of plates called throttle plates or butterflies. A throttle body
is not just part of a carburetor, fuel injection also needs a
throttle body to control airflow. In a carburetor, the throttle body
is usually part of the base plate, but there are a few carbs that
incorporate it into the main body.
Vacuum Secondaries
Vacuum secondaries
rely on a vacuum signal from the primary venturi
to open the secondaries. The vacuum is routed to a diaphragm that
opens the secondaries through some kind of linkage. There is a spring
to control the opening rate, which can be changed to alter the
opening rate.
Venturi
Carburetors all work on one
basic principal, the venturi. A venturi is a very simple device that
changes air pressure by changing the velocity of the air. As velocity
increases, pressure drops and as velocity decreases, pressure rises.
A Venturi is basically an air passage that has a shape similar to an
hourglass. As air travels through the narrow part of the venturi,
it's velocity increases and the pressure drops. If there is a fuel
passage at that low pressure point and the fuel reservoir is at a
higher pressure, fuel will be pushed into the venturi. Simply put, a
carburetor needs a pressure differential to flow fuel; a venturi is
one way to change pressure.
Venturi Booster
Also called a booster
venturi, is just another smaller venturi
situated in the main venturi to further decrease the air pressure.
The pressure drop is also known as vacuum signal, the booster venturi
boosts the signal at the carb. The term signal makes it a little
easier to understand. A strong signal means that there is plenty of
pressure drop to make the carb work, and a weak signal is just the
opposite. A booster venturi will help increase the signal to make the
carb more responsive at the cost of a little restriction to air flow.
How much restriction it is depends on the type of booster. There are
several different types for different applications, but in general,
the more it boosts the signal, the more restrictive it will be.
Any mechanic can tell you
that you won't learn about a carburetor from a picture.
Go find an old junk carb,
take it apart, and put it back together a few times Mess around with
it. Sort and classify all the pieces and then put them back
together. When you've done all that, then you begin to truly
see how the carburetor works . . .
it worked for me
Maintenance
Tips/Suggestions:
Replace the fuel
filter once a year. If your car demonstrates a loss of performance or
fuel economy, have the engine performance evaluated by a good shop
with qualified technicians. The carburetor's health will also be
checked out at this time. Other symptoms of carburetor problems
include hard starting, stalling, hesitation, rough idle, black smoke
from the tailpipe, or failing an emissions test. If you experience
any of these problems, have them checked out at once to avoid more
costly repairs.
The objective of this Web Page is to familiarize you with basic auto maintenance
- in some common emergencies - not to make you an expert in auto mechanics
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I am in no way, shape, or
form telling you to do this yourself. Your results may vary. If
something goes wrong, it is not my fault! These are just guidelines.
