Automotive Carburetors
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Though the use of a carburetor was discontinued in production automobiles in the middle
1990's, being replaced by electronic fuel injection, carburetor are still used in racing applications.
Nascar's continued use of the Holley 4150 four-barrel carburetor is an example of this.
The job of the carburetor is to supply and meter the mixture of fuel vapor and air in
relation to the load and speed of the vehicles engine. Because of engine temperature, speed, and
load, perfect carburetion is difficult to obtain. When a cold engine is first started, a richer
than normal air/fuel mixture is needed. When the engine reaches operating temperature, the fuel is
easily vaporized and the rich fuel mixture is not needed or wanted. The engine will not run as well
with a rich fuel mixture after the engine has warmed up.
When the engine is a idle or at low speeds, another problem must be overcome by the
carburetor, a richer mixture is needed than when it is at medium speeds. Then, when maximum power
is needed, the air/fuel mixture must be as rich as possible. The problem in designing carburetors
was the fact that the airflow rate through the carburetor changes considerably. This is the result
of changes in engine speed. At low speeds, the airflow through the carburetor is at minimum. At
maximum engine speed, airflow will be 100 times greater.
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Air/Fuel Ratio
- The best economy is obtained by a mixture of 1 part gasoline to between 16 to 17 parts of air.
- For quick acceleration and maximum power, a richer mixture of about 1 part gasoline to 12 to 13 parts of
air is needed.
- For idling, a richer mixture also needed.
- When starting a cold engine, an extremely rich mixture is needed.
Carburetor Principles
- To cause a liquid to flow, there must be a high-pressure area, which, in this case, is atmospheric
pressure, and low pressure area.
- The low pressure is less than atmospheric pressure. Low pressure is referred to as vacuum.
- Since atmospheric pressure is already present, a low pressure area (vacuum) can be created by air or a
liquid through a venturi.
- The downward motion of the engines piston creates this low pressure.
- The difference between this low pressure within the cylinder and atmospheric pressure outside of the
carburetor causes air and fuel to flow into the cylinder from the carburetor.
The Venturi
- A venturi is a specially designed section of pipe, tube or line.
- The area of the tube at the center is reduced in diameter.
- The same volume of air or fluid flows through all section of the tube.
- Therefore, the speed, or velocity, of the air or fluid must increase as it passes through the reduced
diameter.
- The venturi not only increases the velocity of the flow of air that passes through it, but it also produces
a low pressure at it point of reduced diameter.
- The outlet of a fuel jet is placed at that point. Fuel is drawn from the jet and mixes with the passing
air.
- This mixing of the fuel with air is know as vaporization, and it closely resembles the action of a spray
gun.
- The difference between the pressure at the venturi and atmospheric pressure on the fuel in the float bowl
causes the fuel to flow.
Carburetor Circuits
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Choke Circuit
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- The purpose of the choke circuit is to provide a richer than-normal fuel mixture for a cold
engine.
- A richer mixture is needed because some of the fuel is condensed on the walls of the cold
carburetor. Therefore, a rich fuel mixture compensates for the lack of vaporized fuel.
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Float Circuit
- Fuel in the carburetor must be maintained at a specified lever under all operating
conditions. This is the function of the float circuit.
- The needed fuel level is maintained by the float.
- When its attached lever forces the needle valve closed, the flow of fuel fro the pump is
stopped.
- Then, as soon as fuel is discharged from the float bowl, the float drops. The needle valve opens
and fuel again flows into the bowl.
- In this way, the fuel is level with opening of the main discharge nozzle.
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Transfer Circuit
- When the throttle is open a little, the flow or air is too limited for the venturi to discharge
fuel from the main nozzle.
- However, with the increased movement of air through the carburetor, more fuel must be supplied in
order to maintain the correct air/fuel mixture.
- To supply the needed fuel during this stage, another port is positioned slightly above the closed
position of the throttle valve.
- A soon as the valve opens a small amount, the port will be exposed to manifold vacuum.
- This will cause fuel to flow from this port also, and the needed fuel will be delivered.
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Main Metering Circuit
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- As the throttle is opened, vacuum at the venturi increases and fuel starts to flow from the main
nozzle, which is centered in the venturi.
- Fuel is discharged from the nozzle during part-throttle and full-throttle positions.
- As the airflow through the carburetor increases, the flow of fuel also increases at a faster
rate.
- This is because the density of the fuel does not change, while that of the air does. Consequently,
the mixture in a simple carburetor will be too rich under wide open throttle.
- Since the correct air/fuel on a simple carburetor would be supplied at only one position of the
throttle, steps must be taken to provide the correct mixture of all positions of the throttle
valve.
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- A metering rod varies the size of the carburetor jet opening.
- Fuel from the float bowl is metered through the jet and the metering rod within it.
- Fuel is forced from the jet to the nozzle extending into the venturi.
- As the throttle valve is opened, its linkage rises the metering rod from the jet.
- The rod has several steps, or tapers, on the lower end.
- As it is raised in the jet, it makes the opening of the jet greater in size.
- This allows more fuel to flow through the jet to the discharge nozzle.
- The metering rod must keep pace with the slightest change in the throttle valve position son that
the correct ari/fuel mixture is obtained through all engine speeds.
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The power circuit consists of a step-up piston fastened to metering rods.
- When high vacuum develops in the intake manifold (part throttle operation), atmospheric pressure holds
the step-up piston down against spring pressure.
- This, in turn, holds the metering rods down in the main metering jet, closing the jet.
- When no vacuum is in the intake manifold (wide-open throttle), the difference in pressure above and
below the piston is the same.
- The piston is then moved up by spring pressure and the rod is raised out of its get.
- In this way, additional fuel is allowed to flow through the jet for maximum power.
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Accelerator Pump Circuit
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- When a throttle valve is opened quickly to produce rapid acceleration, the carburetor fuel mixture
tends to become too lean and a hesitation occurs.
- This results from the fact that the fuel is of greater weight than air. When the accelerator is
opened suddenly, the flow of fuel will lab behind the flow of the air.
- To supply the extra fuel needed to overcome this situation, an accelerator pump is part of the
carburetor design.
- This pump is operated by the throttle linkage.
- The accelerator pump circuit generally consists oh the following:
- A pump well
- A plunger, which is mechanically actuated by a lever connected to the throttle shaft.
- An intake check ball located in the bottom of pump well to control passage of fuel from the bowl to the
pump cylinder.
- A discharge check ball located in the discharge passage to prevent fuel in the accelerator pump well
from being siphoned into the air stream.
- A discharge nozzle (pump jets) located in the throat of the carburetor.
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As the throttle is opened, the pump plunger moves downward.
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The downward travel of the plunger forces fuel past the discharge check ball.
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Fuel is then supplied to the pump cylinder through the intake check ball at the bottom of the well.
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This check ball permits a supply of fuel to reach the cylinder, but closes on the down stroke of
the plunger to prevent fuel in the well from being pushed back into the float bowl.
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Single-Barrel Carburetor
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- A carburetor is classified by the numbers of throats, or barrels.
- A single-barrel carburetor has one outlet to the intake manifold.
- This is designed to take care of all the needs of the engine under
all conditions.
- A single-barrel is used on engines having a maximum or six
cylinders.
- In the two-barrel carburetor there are two outlets to the intake
manifold.
- These units are two single-barrel carburetors in one, with two complete
idling circuits, two high-speed circuits, two power circuits, Two accelerator discharge passages, and
two throttle valves.
- However, they have only one float system.
- With a two-barrel carburetor, each barrel supplies alternate cylinders
in the firing order.
- In a six-cylinder engine one barrel supplies cylinders 1, 3, and 2.
- the other barrel supplies cylinders 5, 6, and 4.
- In the four-barrel carburetor, there are four openings in the intake
manifold.
- Some systems, such as the float system, may be common to all four
barrels
- In four barrel designs, half of the carburetor operates on a two-barrel unit
during light loads and cruising speeds.
- The other half of the carburetor is supplemental for top speed and
full-throttle.
- The two barrels that supply fuel for light loads are known as the primary
side.
- The supplementary two barrels are know as the secondary side.
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Congratulation, if you have read this far you are well on your way to
understanding carburetors, remember, they all operate on the same principle, even though they may look
physically different they all have the same parts and operate the same.
Now you should go find some old carburetors, take them apart and see if
you can identify the part that make them work. Look for them on old cars just sitting around,
salvage yards, you know, because your a car guy/gal if you are interested in carburetors. Remember
they are all the same, some just have more parts then others. This applies to all carburetors
whether it is for a lawn mower or a large V-8 car engine. Good luck and have fun!
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