The Carburetor System
A carburetor is used to measure the amount of incoming air and meter out the proper fuel
or fuel/air ratio to the cylinder intake valve.
Most carburetors used are the float type. This means the carburetor has a bowl filled with
fuel to a level that is regulated by a float attached to a needle valve. Fuel enters the bowl through
a strainer which filters the fuel from any containments or particulate matter. As the fuel level
increases, the float rises and the needle valve, which is attached to the float by a lever, closes and
shuts off the fuel flow until the float level drops again.
Air enters the carburetor and travels through a venturi. The venturi accelerates the airflow
and causes the pressure of the air to drop. A nozzle is placed in this area of low pressure and is
connected to the fuel bowl. The low pressure creates suction on the nozzle and fuel is discharged
into the air stream. As the fuel is discharged it also vaporizes.
The amount of suction on the nozzle is regulated by the amount of airflow going across
the nozzle. This amount of airflow is controlled by the throttle valve (“butterfly” valve) located
downstream of the venturi and discharge nozzle. As the throttle valve is closed by moving the
throttle cable, the airflow is reduced. As the throttle cable is opened and airflow and suction on
the discharge nozzle are increased.
The mixture control on the carburetor regulates the amount of fuel coming out of the
discharge nozzle. The throttle regulates the amount of suction, and the mixture controls the fuel
amount, which allows for the proper adjust of the fuel-to-air ratio. (Shipe, 2015)
The Carburetor System
The are several advantages to a carburetor system. These advantages include the ease of
maintenance. They are also simple to design and economical to manufacture with replacement
parts readily available. However, there are disadvantages that should be considered.
The fact that carburetors cannot provide perfect fuel/air ratio on a consistent basis, nor
can they control the fuel waste effectively has forced some to look for an alternate fuel delivery
system. As with any mechanical part, they are subject to wear and tear and therefore must be
serviced periodically.
The greatest threat a carburetor poses (particularly in aviation) is that of carburetor icing.
Under certain conditions, carburetor ice can occur. This takes place as the air flows through the
venturi it creates a drop in air pressure which causes the air to cool rapidly and draw heat away
from the venturi. If there is a high amount of humidity in the air it can freeze and hinder the fuel
flow to the engine. Even if the outside air temperature is way above the freezing point.
Aviators must also be aware of carburetor icing as they descend from higher altitudes.
The only way to defeat carburetor icing is by using carburetor heat. Typically, heat is drawn
from the exhaust system to prevent ice from forming as well as dissipating any ice formation
within the carb by employing the carb heat lever found in most aircraft. Caution and proper
training should always be considered when using this application.
