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Automotive Training Manual - Carburetion

This is the text of a booklet dated 1965.


This carburetor training manual is intended to explain the "how" and "why” of carburetor theory as it is applied to one of the most popular carburetors in use today -- the Rochester two-barrel carburetor.

The manual itself is divided into two chapters dealing with:

I. Basic carburetor theory.

II. Carburetor theory as it is applied to the Rochester two-barrel carburetor.

Rochester 2GC carb
Rochester Carburetor With Automatic Choke



Engines will not run on "liquid" gasoline. A metering device must be used to mix fuel and air in the proper proportions to accommodate all different types of starting and driving ranges.  

Example: Cold or hot starting, idle, part throttle, acceleration and high speed operation.  The carburetor performs this function.

The formation of the air/fuel mixture consists of subdividing or atomizing the fuel and mixing the finely divided particles of fuel with air. Atomization is accomplished by adding air to the liquid fuel as it moves through the carburetor passages and then spraying this air/ fuel mixture through nozzles or jets into a stream of moving air which is flowing into the intake manifold. This air/fuel mixture must then be converted into vapor before it is introduced into the combustion chamber of the engine. The vaporization is assisted by the heat of the intake manifold.

Variations in speed and load conditions demand a different volume of air and a corresponding difference in fuel volume. The mixture ratio, however, must be kept within flammable limits if combustion is to occur. The primary object of the carburetor is to produce the desired ratio for any operating condition.

The air/fuel mixture ratio is roughly 15 parts of air to 1 part of gasoline by weight. Represented as volume, this would be approximately 9,000 parts of air to 1 part of gasoline. The proportions will vary under different conditions of speed and load.

The potential energy contained in 1 gallon of gasoline is 95,083,000 foot pounds. If all this energy could be converted into work without any loss, the modern motor vehicle would go well over 450 miles on a gallon of gasoline. However, due to weight, heat losses and friction of moving parts, only a small percentage of the heat energy of gasoline is converted into available power by the engine.


next: Vacuum and Atmospheric Pressure


Table of Contents


Basic Carburetor Theory and Terms

Vacuum and Atmospheric Pressure

Principle of the Four Stroke Engine

Venturi Principle - Principles of Carburetor Operation

Theory of Operation

Main and Power Circuits

Accelerator Pump System

Choke Circuit