Carburetors
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A carburetor is a mechanical device which mixes air and fuel together for
combustion in an engine at approximately the correct air fuel ratio.
Engine requirements vary widely, depending on weather, temperature, load,
and speed. And, there are as many other variations as there are types and
sizes of engines, so carburetor design is dependant on a number of different
factors.
Carburetors have a hard time compensating for all of these factors when
compared to fuel injection.
However, a carburetor can be prepared and tuned to create both good
drivability.
They are widely available through performance retailers and are relatively
inexpensive.
With the wide spread application of fuel injection, tuning of carburetors has
become somewhat of a ‘lost art’.
Carburetors
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The fueling requirements vary depending on the current
operating conditions.
For example:
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While cruising at a steady speed, an air/fuel ratio of around 14.7:1
(known as ‘stoich’) provides all the power the engine needs to propel the
vehicle. If the engine can tolerate it, running leaner, maybe 16:1, will
provide increased fuel economy.
WOT (Wide Open Throttle) demands a rich air/fuel ratio. Generally, the
engine will produce the most power at an air fuel ratio between 12:1 and
13:1. The exact ratio depends on operating conditions and engine design
and is arrived at by experimentation on the dynamometer.
When the engine is first started on a cold morning, the air/fuel ratio
needs to be exceedingly rich due to ‘fuel dropout’ onto the cold manifold
surfaces. A 6:1 air/fuel ratio would not be uncommon.
Carburetors
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A Matter of Matter
Matter can exist in three different states: solids,
liquids, and gas.
 Two of these states, liquid and gas, as known as
fluids.
 This means that they will flow readily from one
container to another.
 For example, water is a fluid and will flow easily
through a garden hose. Air is a fluid, and will flow
easily through an air hose in your shop.
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Carburetors
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What causes flow?
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Movement of any fluid is the direct result of difference in pressure.
Direction of flow is always towards the point of least pressure.
Water flows through a hose because the end is exposed to atmospheric
pressure. The other end of the hose is subject to the greater pressure in
the water line.
An engine is an air pump.
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Every time a piston goes down on an intake stroke, it creates a vacant
space.
Atmospheric pressure tries to fill this vacant space by flowing through the
carburetor.
The rate of flow is controlled by the driver, when he selects the desired
throttle opening.
Carburetors
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Vacuum versus Low Pressure
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Vacuum is just another word for low pressure, or pressure that is lower
than atmospheric.
Vacuum is measured in inches of Mercury (“Hg).
There are 3 different ‘types’ of vacuum found in an engine.
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Manifold vacuum – vacuum port which is connected to the intake manifold
between the intake valve and throttle plate in the carburetor. At idle, a
vacuum gauge hooked up to a manifold vacuum port will show a high
vacuum. Once the throttle is opened, the vacuum will drop to near
atmospheric pressure.
Ported vacuum – vacuum is picked up from a port directly above the throttle
plate. If a vacuum gauge were hooked up to this port, at idle there would be
no vacuum. However, when the throttle is cracked open the gauge would
begin to register a vacuum. If the throttle is opened to WOT, the gauge will
again go to near atmospheric pressure.
Venturi vacuum – vacuum is picked up from a port in the carburetors venturi.
The amount of vacuum that registers on a gauge is proportional to the
amount of air flowing through the venturi.
Carburetors
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Venturi Principle
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The most important law of physics involved in carburetors
was discovered by Daniel Bernoulli in 1775. It deals with the
venturi.
The venturi constant.
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The ‘constant’ condition in a venturi is made up of two primary
factors, velocity and pressure.
The two factors, multiplied together, form the constant for any given
condition and a particular time.
Pressure times velocity equals the constant.
The factors of pressure and velocity depend on the pressure
differential that is causing the flow. The constant will be different for
different flow conditions.
If the constant does not change, if velocity goes up, pressure will go
down. If pressure goes up, velocity goes down.
Carburetors
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Sizes and Speeds
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When the engine draws air through the carburetor, the speed of flow is
determined by engine demand.
The speed of flow governs the amount of fuel that is drawn into the
cylinders.
When air flows through the venturi, it speeds up.
That is because the same amount of air is flowing through the barrel of
the carburetor, including the restricted venturi portion.
The venturi is a narrow portion of the carburetor barrel. In order for the
same amount of air to travel through this portion, it must speed up.
Daniel Bernoulli proved that as air, or any other fluid, passed through the
narrowest part of a venturi, it increases in speed, but decreases in
pressure.
This is contrary to the appearance of the tube. It would seem that when
the air goes through the narrow part of the venturi, the air would be
compressed and the pressure would rise, but this is not true.
Carburetors
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Six basic systems in a carburetor
Float
 Choke
 Idle
 Low speed
 Acceleration
 High speed
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Each has a specific function
 Some of the functions overlap
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Carburetors
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Float circuit
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The float bowl is a storage reservoir which maintains a precise level of
fuel.
This assures there will be an ample supply of fuel for the various fuel
delivery requirements of the engine to meet any operating condition.
The fuel is supplied to the float system by the fuel pump, which supplies a
constant fuel pressure (usually 3 – 9 psi depending on carburetor model,
consult owners manual for your spec.)
Needle, Seat, and Float
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The opening and closing of the needle and seat is controlled by the float and
fuel pressure.
Float is hollow and rides on the fuel.
As long as the fuel level in the bowl is below where is should be, the needle
and seat is open and fuel flows into the bowl.
When the amount of fuel in the bowl reaches the proper level, the rising float
shuts the needle and seat and blocks further fuel flow into the bowl.
As the fuel in the bowl is consumed, the needle and seat open again and the
cycle repeats.
Carburetors
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Float level is very important
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If set too low, the engine will run lean.
If set too high, the engine will run rich.
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Also possible for fuel to flow down throat of carburetor after car is shut off
and ‘flood’ it. The result will be a hard or impossible to start engine.
Float bowl has a vent which goes to the atmosphere. It is sometimes
called a ‘bowl vent’ or a ‘balance tube’.
It is possible for a hollow float to get a hole in it. This condition is called
a ‘sunk’ float and will result in an extremely flooded engine. Plastic floats
that are solid are made to prevent this.
‘Percolation’ is when fuel boils in the float bowl after the engine
is shut off and heat soaks. This is usually seen as a hard start
after the engine has sit ten or more minutes after the engine is
shut off.
‘Vapor lock’ is when fuel boils in the fuel line, and the pump can
no longer supply sufficient fuel to fill the bowl.
Carburetors
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Idle system
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Essentially, a passage that bypasses the closed throttle plate.
Air is mixed with fuel through and ‘idle air bleed’ and travels
down to the idle mixture adjustment screw.
Idle air bleeds ensure that the air is volatilized enough to be
distributed to all cylinders.
Idle mixture adjustment screws ONLY adjust mixture at idle.
It does not affect the mixture off idle at all.
Any mixture adjustments off idle must be done by changing
jets/metering rods.
Carburetors
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Low speed system
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Low speed system can be considered a bridge between the
idle system and the high speed system.
As throttle is opened slightly, vacuum moves up carburetor
barrel slightly above throttle plate.
A ‘transfer port’ is slightly about the throttle port and works
similarly to the idle system, except there is no adjustment.
As high speed system comes into operation, both the low
speed system and the idle system are phased out and no
longer supply fuel.
Carburetors
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Acceleration System
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When the throttle is opened suddenly, from the low speed or
idle position, a couple of things happen that prevent the
engine from getting the richer mixture it needs. If it does
not, the engine will stumble and may stackfire. This may
result in a carburetor fire.
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Fuel is no longer drawn from the idle and low-speed systems due to
loss of a low pressure area near them. This leans the mixture.
The high speed system cannot supply the extra fuel immediately.
Since this is a mechanical device, it takes a moment for fuel to get
flowing. This leans the mixture temporarily.
As the pressure inside the manifold suddenly increases, fuel ‘drops
out’ of the air/fuel mixture and condenses on the manifold walls.
This leans the mixture.
Carburetors
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Therefore, fuel must be force-fed into the intake air stream
until the high speed circuit can come into operation.
An ‘accelerator pump’ is what does this.
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Located in a separate well in the float bowl.
Sprays fuel directly into the air stream to make up for lean mixtures
during ‘transient’ conditions.
At idle, the throttle linkage pulls the pump to its highest position.
When accelerator pedal is depressed, the linkage releases the pump
operating rod. This allows the accelerator pump spring to actuate the
pump.
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The throttle linkage does not depress the pump, the spring does. It just
allows it too.
If the throttle is quickly opened to WOT, fuel may flow from the
accelerator pump nozzles for several seconds.
Generally only works during the 1st half of throttle travel.
Carburetor
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High Speed System
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As air requirements increase, the idle and low speed systems are phased
out and the high speed system comes into play.
The high speed system is the main metering system and is what uses a
venturi.
A basic system includes a discharge tube, venturi, main well passage, and
main metering jet.
The discharge tube extends from the main fuel well to the narrowest part
of the venturi.
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It is possible for a carburetor to have a ‘double venturi’.
The fuel in the main well is at the same level as the fuel in the float bowl
as long as pressure remains equal.
Once significant air starts flowing through the venturi a vacuum is
developed at the discharge tube.
Since the float bowl has atmospheric pressure above it, fuel will begin
flowing out the discharge tube.
Carburetors
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Choke System
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Choke provides the rich mixture needed to start a cold
engine.
A ‘choke plate’ partially blocks off the top of the carburetor.
This causes a vacuum in the idle, low speed, and high speed
systems, and causes fuel to flow from all which causes a rich
mixture.
Choke must be gradually opened as engine warms up.
A manual choke uses a cable and knob to open and close the
choke.
An automatic choke is either electric or heated by the
temperature of the engine.
Carb will also have a ‘fast idle cam’ which increases the idle
while the choke is on for easier running when cold.
Carburetors
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How to start a carbureted engine:
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Unlike a fuel injected engine, there is a special starting
procedure for carburetors.
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Pump gas to the floor once and release pedal all the way.
Crank engine to start.
If engine does not start, pump pedal once more and crack throttle
about ¼ open and crank engine until it starts.
If engine does not start, it either has not gotten enough fuel or it is
flooded.
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Pump the gas three times and see if it will start.
If it doesn’t, it is likely flooded. Press the gas all the way to the floor
and crank the engine until it starts.
Lower volatility of modern fuels and different distillation curve has
made fuels for carbureted vehicles less than optimal, especially when
it comes to starting.
Starting a carbureted car may require practice.
Carburetors
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Tuning carburetors
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There are many good brands of carburetors on the market
with a good variety of supporting parts.
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Holley
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Edelbrock
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Now makes the ‘AFB’, which was formerly made by Carter, Magnetti
Marelli, and Weber.
Weber
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Make wide variety of 2 and 4 barrel carbs
The standard in racing
Side draft Weber
4 Webers on V8
Quadrajet
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Installed on many, many GM vehicles.
When tuned correctly they are an excellent performance carburetor
There is a lot of support for them
Carburetors
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Tuning carburetors
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First, rough tune idle so it won’t stall.
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If engine idles, bump idle up to about 1000 rpm.
Spray a short burst of carburetor cleaner into the carb. If idle increases, the
mixture is lean. If engine bogs down then mixture is too rich.
Turn off engine. Turn idle mixture screws in, counting the number of turns
it takes until the mixture screw stops turning. Do not force it. Record
number of turns.
If carb has multiple mixture screws they should be turned out an equal
number of turns.
If mixture was too lean, turn screws out ¼ turn more than they were
previously.
If mixture was rich, turn screws out ¼ turn less than they were.
Spray another burst of carb clean in carburetor and see what happens.
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If engine is still too lean, back off all idle mixture screws an equal number of
turns ¼ turn at a time.
If engine is too rich, do the same but turn the screws inward.
Some Holleys work in reverse! If you are not getting the desired results, try
going the opposite way with the screws.
The same procedure is used when setting idle to final spec.
Carburetors
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To tune a carburetor properly you need an emissions
analyzer (4 or 5 gas analyzer).
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What goes in, must come out.
Lets you know how efficient combustion is.
Air/fuel ratio monitors do not seem to work well for
tuning carbureted cars.
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Great for precisely tuning load/rpm points on fuel injection.
Since a change in jet size in a carb affects fuel metering at all
rpm you do not need something so precise.
Carburetors
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4 gas analyzer
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CO (Carbon Monoxide)
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HC (Hydrocarbons)
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Measured in percentage
Lean mixtures will have nearly no CO
CO will go up progressively as mixture gets richer.
Measured in PPM (parts per million)
Lean mixtures will have high HC
Correct mixtures will have low HC
Overly rich mixtures will have high HC (and high CO)
CO2 (Carbon Dioxide)
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Measured in percentage
Measure of combustion efficiency
When tuning non-catalyst vehicles I usually ignore this
Carburetors
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Setting idle mixture with HC and CO
Set idle to desired RPM
 Check exhaust gases with warm engine
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CO should be 3-4%. The higher the CO the richer the
mixture is. So if the CO reading is 5%, you need to lean
out the mixture by turning in the screws.
 HC - On a stock street vehicle with no cats, HC at idle
should be below about 300 ppm. With a cam with a lot
of overlap, HC can be as high as 1000 ppm.
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Carburetors
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Tuning mixture at cruise (high speed circuit)
with gas analyzer.
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Bring engine up to 3000 rpm.
CO should be between 1.5% and 3% CO
 HC should be between 0 and 300 ppm
 If CO is above 3%, carb needs smaller jets, or larger
metering rods.
 If CO is below 1.5%, carb needs larger jets, or smaller
metering rods.
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