There is one element without which an engine can’t
operate—no matter what kind of engine it is—FUEL. In our
case the fuel is gasoline and oxygen. Without oxygen-the
fuel cannot be burned to release its energy.
So how do both of these elements make their way into the
engine? There are two ways, one mechanical, one
The mechanical way is through the use of a carburetor. A
carburetor is a device that mixes incoming air and a mist of
fuel just before they enter the engine cylinder. A basic
carburetor has two parts- a throttle body and a fuel bowl.
The throttle body is a valve through which air from outside
the engine passes into the engine and into the cylinders.
There is a flap inside of the valve which opens and closes,
restricting and opening the flow air with the motion of the
“gas” pedal, or more accurately throttle pedal. To make
the engine spin faster, you push down the pedal which
opens the throttle valve and allows more air into the
engine-allowing it to speed up.
The second part is a fuel bowl, which is a small reservoir
holding fuel and is connected to the throttle valve by way
of a small opening or nozzle. Fuel is pulled from the gas
tank and pumped into the fuel bowl where it waits to enter
the engine. The fuel pump for a carbureted engine is
usually mounted on the engine and powered by the
spinning of the camshaft. For a fuel injected engine the
pump is loacted in the fuel tank or along the fuel lines.
During the intake stroke the cylinder creates a vacuum,
which sucks air from the outside into the cylinder-the same
way a bicycle pump does when you pull on the handle. As
the air is sucked in- it passes through a narrowing tube in
the carburetor called a venturi, which accelerates the airflow
and reduces its pressure (see venturi effect below). When
the pressure drops in the venturi a small amount of fuel is
pulled out of the fuel bowl, through a nozzle (referred to as
a jet) into the venturi tube. This fuel then literally boils in
the low pressure environment within the narrowing of the
venturi. This essentially means it vaporizes and this vapor
is mixed with the air and pulled into the cylinder.
On many Carburetors there is a small piston between the
fuel bowl and jet called the accelerator pump. When the
throttle is depressed, this pump pushes more fuel through
the jet so that the engine accelerates quickly without
waiting for the vacuum to slowly suck it out. Without the
pump, acceleration would be very slow.
Creating a mist is a critical step, because gasoline only
ignites when it is suspended in the air, like a mist or
fumes. If liquid gasoline were simply poured into the
cylinder it would not ignite. When too much gasoline
enters the cylinder, it is referred to as a flooded cylinder.
The gas collects into a liquid and will not ignite. Only
when just enough gasoline is misted into the air will it
The flow of gasoline into the throttle body is controlled by
changing the size of the nozzle or jet. A larger nozzle
flows more fuel, a smaller one flows less. Matching the
right size nozzle to provide the perfect mist of fuel for the
size of the cylinder is the primary adjustment on a
carburetor. The other main adjustment is the idle.
Idle is the speed at which an engine is running without
pressing on the gas—usually around 500-700 rpm. A small
amount of air and fuel is allowed to flow through the
carburetor even with the throttle closed (through a
separate small nozzle) to keep the engine operating at
standby. An engine runs at its idle speed when a car is
parked, in neutral or at a red light.
The idle speed is adjusted by changing the size of the
nozzle-usually by way of turning an adjustment screw on
the carburetor. A good idle speed is fast enough to keep
the engine from stopping or “stalling,” but slow enough so
as not to waste fuel or strain the engine. As we have said
before a good speed is just over 500 RPM, as most engines
will stall at any slower speed.
Single barrel carburetor diagram
Venturi Effect: Simply put, air flowing through a tube that
is narrowing goes faster the narrower it gets. As each air
molecule moves faster it results in a drop in pressure at the
narrowing. In contrast, air flowing through a widening
tube gets slower, and its pressure increases.
What it does:
Air and other fluids/gases flow from high pressure to low
pressure. Thereby the low pressure created by the
narrowing venturi effectively sucks fuel into the barrel of a
carburetor (similarly it sucks paint out of a paint bottle and
into an airbrush, or out of spray paint can. Within the low
pressure of the venturi the fuel or paint boils and vaporizes
creating the familiar mist. This is because the boiling point
of a material increases and decreases with pressure. (e.g.,
the water in a car radiator doesn't boil and steam until the
cap is removed and the pressure reduced.)