Counter Pro
Course - Carburetor/Fuel Pump Training 5
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Throttle plate action. |
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The speed of the incoming air through the venturi is governed by the throttle plate (or
"butterfly" as it is sometimes called). The further the throttle plate is open, the greater the
amount of air that gets inside. The speed of the air determines the difference in air pressure
between static and moving air. Similarly, the more air that gets in, the greater the amount
of fuel pushed into the system.
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Jets. |
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The limiting device for air and fuel is the "jet." The fuel jet screws into the bottom of the
well; the air bleed jet screws into the top of the well. When the jet is not a screw-in type, it
may be pressed or glued in with high-tech adhesives. The jet has a small, high-precision
orifice or hole in the center. To flow more fuel or air, a jet with a larger orifice is used. To
flow less, a smaller orifice is used.
To meter the correct amount of air and fuel into the engine for its various working
conditions, the engineers work with this mixing of air and fuel to create an emulsion by
using the jet orifice size and the differential in pressure between the fuel side and intake air
side of the carburetor's systems.
The "carburetor system" refers to the air/fuel delivery system as a whole. However, a
carburetor is also an assembly of subsystems -- each of which is designed to cope with a
different problem of mixing air and fuel. The systems can be described as follows:
1. Choke system, including fast idle linkage
2. Float system
3. Idle fuel system
4. Off-idle system
5. Main metering system
6. Wide-open throttle (WOT) or power enrichment system
7. Acceleration enrichment system
These systems are typically located in one of three major carburetor assemblies:
1. The air horn, which is found on the very top of the carburetor and secured with screws
to ...
2. The center section, which holds most of the fuel control mechanisms and components
and is attached with screws to ...
3. The throttle body section, which is a mounting section for the throttle or butterfly valves
that are operated when the driver of the car puts his foot down on the accelerator. The
position of the throttle plates controls air intake speed and volume through the carburetor
and into the manifold.
It is a fact that any fault with the carburetor (other than a cracked casting which is not
visible from the exterior surface of the castings and is a rare occurrence, indeed), can be
positively identified before a carburetor is removed for a tune-up.
Thus, if the carburetor was not the cause of the problem, a carburetor tune-up obviously
will not cure the problem.
However, things can -- and do -- go wrong with the carburetor. Here are some of the
problems:
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Dirt. |
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Dirt is the major enemy. It comes from both the air and the fuel and can damage sealing
surfaces, such as the needle valve and seat, and clog fuel and air passages.
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Heat and vibration. |
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Carburetors have small parts which can be secured to the carburetor casting with high-tech
adhesives. If the engine overheats, these adhesives can loosen and the "glued-in" parts can
be dislodged by vibration. Or, they may leak and upset various air/fuel ratios.
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Fuel erosion. |
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Over extremely long periods of use, the movement of fuel through the various metering
devices can cause erosion to the metal and changes in metering orifice size.
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Contamination. |
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Some gasoline additives can seriously damage the "soft" parts of the carburetor, such as the
gaskets and synthetic rubber diaphragms. Methyl alcohol is the major offender today.
However, there are other additives which, when used in too high a concentration, can also
cause failure of soft parts.
According to the above, age, unusual working conditions and poor fuel quality can all
cause a carburetor tune-up to be necessary.
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Recap. |
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The purpose of the carburetor is to mix air and fuel together in a mist which can be used
by the engine. The ratio of air and fuel is critical, and ranges between 8:1 and 18:1. The
ideal or "stoichiometric" ratio is 14.7:1. Static air pressure is used to push fuel and air
through the system. The pressure differential required to allow fuel and air to move
through the jets and wells in the carburetor is caused by fast moving air which is created
by a "squeezing" action at the venturi inside the carburetor. There are seven basic sub-
systems in the carburetor required to handle starting, fuel level, off idle, main metering or
"cruise," WOT or maximum power, and acceleration conditions.
Dirt, heat and vibration, "old age," and fuel contamination are the primary enemies of
the carburetor and its various subsystems.
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