The Fuel Injection about Carburetor (2)



Cost is another major reason for choosing
carburetors over fuel injection. Even though most cars today use fuel
injection, carbs are still plentiful and relatively cheap. Used ones can be
picked up at swap meets, via other car enthusiasts, speed shops or on the web.
In addition, aftermarket manufacturers, including Barry Grant, Edelbrock and
Holley, sell a huge range of brand new carburetors, designed for everything
from mild street driving to all out drag and road racing. In addition, there
are numerous specialists out there to assist with tuning and rebuilding


How Carburetors Work

In very basic terms, think of a carburetor
as a length of pipe. At one end of the pipe is moveable flap that opens and
closes – the throttle plate. This controls the amount of air entering the pipe.
Further down, the pipe narrows inward (the venturi). On this section of the
pipe there’s a small hole, housing a jet, which squirts fuel into the venturi.
Automotive carburetors use three cycles or circuits to deliver fuel: idle, part
and full throttle. Depending on how much the throttle plate is open, a certain
amount of fuel is needed in proportion to the air rushing in order to allow the
spark to ignite the mixture and fire the engine. The theoretical ideal or
Stochimetric ratio for complete combustion is 14 parts air to 1 part of fuel
(14:1); however, in practice, that is rarely achieved with carburetors.


A cold engine requires more fuel to operate
than one that’s warmed up, since fuel is the more combustible of the two
elements that need to be ignited by the spark. In the simplest of terms, the throttle
plate, besides controlling the amount of air flowing into the carb, also
controls the amount of vacuum in the venturi on both the top and bottom sides
of the carb. On the bottom side, when the plate is closed, there’s a buildup of
vacuum. By placing a secondary jet right below the throttle plate (the idle
jet), the fuel can be drawn down the venturi more quickly and into the manifold
and combustion chamber. This results in a richer air/fuel mixture, allowing the
engine to start more easily when cold. (In addition, a manually-adjustable
mixture screw can also alter the air/fuel ratio under certain operating
conditions). In most automobiles, a secondary, moveable plate, called the choke
plate, is mounted below the throttle plate. When the engine is cold, it fully
blocks the flow of air through the venturi from the top side of the carb. As
the engine warms up, the choke plate is pulled back, increasing airflow and
reducing the flow of fuel from the idle jet. Chokes on automotive carburetors
are either operated manually (often via a lever), or automatically, via an
electrical wire and heating element that’s linked to the ignition system and
fuel cut-off valve. As the electrical current warms up, the heating element
expands, gradually opening the choke plate. The level of heat passing through
the electrical current determines how much the choke is opened (more under
warmer conditions, less under colder ones). Once the engine warms up, the carb
switches from the idle circuit to the part throttle circuit, relying on the
main jet to deliver the fuel. At full throttle, the main jet and air bleed jet
(mounted at the top of an emulsion tube) provide fuel delivery. (The emulsion
tube is a bit like a straw, which pre-mixes the air/fuel before it reaches the
venturi and combustion chamber, to reduce the risk of detonation.)


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