Mechanics

Carburetor

Article describes how an automotive carburetor works.

A carburetor is an essential part of older model, internal combustion engine. The main function of a carburetor is to mix the fuel and air and feed it into the vehicle's combustion chambers where its ignited and used to thrust the pistons downward inside the engine block. This force against the pistons is what creates power and causes a car to accelerate. The basic physics behind the function of a carburetor is called the "Bernoulli principle" the venturi effect. The Bernoulli principle states that speed of the air is inversely proportional to the pressure. The throttle plate or butterfly of the carburetor manages the amount of air flow that is delivered to the engine.

Holly Carburetor
Carburetor - (appearance may vary)

The velocity of the air flow and the subsequent pressure, gauges the quantity of emulsified fuel that is fed into the air stream. Carburetors use tubes called venturies to achieve this effect. Below the venturi there is a valve called a throttle plate which can be open and closed by the gas pedal. This controls the engine speed by restricting the air flow to the engine and subsequently, the amount of air & fuel mixture that is delivered into the engine. When the gas pedal is depressed, fuel is drawn into the air stream via the venturies and the engine speed increases.

Carburetor Parts

  • Accelerator pump (used to inject fuel into the engine just as the throttle is opened. Accelerator pump also primes the engine at cold starts.)
  • Idle circuit (used to supply a small amount of fuel to the engine at idle only.)
  • Fuel jets (used to meter the raw fuel allowed into the venturies which is then mixed with air.)
  • Throttle plate (used to meter the amount of fuel-air mixture allowed into the engine.)
  • Choke (used to increase the amount of fuel drawn into the engine when cold only. A cold engine needs a rich mixture to run properly. The choke also holds the throttle plate open slightly to increase the idle speed to prevent stalling.)
  • Fuel level float (used to maintain the proper fuel level inside the carburetor.)
  • Fuel filter (used to filter fuel before it enters the carburetor.)
Common Problems

The main problem with a carburetor is dirty fuel, this can cause the carburetor to malfunction. To check a carburetor air/fuel mixture a gas analyzer is needed. This will measure the carbon monoxide, hydrocarbon and oxygen content of the exhaust. During a normal smog inspection the fuel/air mixture is tested and a report is generated. When a tune up is performed the spark plug insulator can indicate a lean or rich mixture. Brownish grey is the desired color and confirms proper mixture. Black and sooty means the mixture is too rich while white to light grey confirms a lean mixture. Visit - Spark plug inspection

Basic maintenance

Changing the air filter and fuel filter will keep dirt/debris from entering the internal workings of the carburetor. Air and fuel filters should be changed at regular intervals. Visit - Fuel filter replacement Visit - Air filter replacment

AUTHOR


Written by
Co-Founder and CEO of 2CarPros.com
35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.


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Article first published (Updated 2013-11-30)