2. Objectives Terms
Students will be
able to list and
explain the
principles of
operation
pertaining to small
engine carburetors.
Carburetor
Air-fuel mixture
Vacuum
Atmospheric
Pressure
Venturi
3. What is a Carburetor?
A carburetor’s
primary purpose is
to produce a
mixture of fuel and
air to operate the
engine.
4. Carburetion
Gasoline engines cannot run on liquid
gasoline. It must be vaporized and
mixed with air in the proper
proportions for varying conditions.
5. Carburetion
The carburetor must
create an air fuel
mixture that is
correct for different
circumstances such
as:
Cold or hot starting
Idling
Part throttle
Acceleration
High speed
operation
6. How does it work?
Air enters the top of
the carburetor and is
mixed with liquid fuel.
The air fuel mixture is
forced into the intake
manifold by
atmospheric pressure
and burned in the
combustion chamber
of the engine.
7. Air-Fuel Mixture
The mixture will vary
depending on the
conditions. The proportion
is given as the number of
pounds of air compared to
the number of pounds of
gasoline.
At normal operating speed
a small engine will use an
air-fuel mixture of about
15 pounds air to 1 pound
of gasoline
8. Pressure Differences
Carburetors work
on the principle of
air pressure
differences. When
discussing pressure
differences we will
talk about:
Vacuum
Atmospheric
Pressure
Venturi
9. Vacuum
An absolute vacuum is
an area completely
free of air or
atmospheric pressure.
Although an absolute
vacuum is not reached
in a small engine, any
pressure less that
atmospheric pressure
is generally referred to
as a vacuum
10. Atmospheric Pressure
Atmospheric
pressure is the
pressure produced
by the weight of air
molecules above
the earth.
A partial vacuum is
produced by the
piston on the
intake stroke.
When the intake
valve opens,
atmospheric
pressure forces air
through the
carburetor to fill it.
11. Venture Principle
A venturi is a
restriction in an air
passage that increases
air speed or velocity.
This increase in
velocity reduces
pressure causing fuel
to be drawn into the
air stream
Particles of fuel are
vaporized by air
rushing through the
venturi.
12. Vaporization
Although the venturi
breaks the fuel into
fine particles, it is
further vaporized by:
The heat of the
engine in the intake
manifold
The swirling action of
the air in the
combustion chamber.
13. Combustion
Cold fuel is difficult to vaporize, this is
why we choke or prime a cold engine
to help get it started.
Over choking or priming can cause
raw fuel to be pulled into the
combustion chamber resulting in
bypass or a condition known as
flooding.
14. Summary
Since a gasoline engine does not
operate on liquid fuel, it is the
responsibility of the carburetor to
provide it with an air-fuel mixture.
The carburetor operates on the
principles of the following:
Vacuum
Atmospheric Pressure
Venturi
