Lesson 1/learning Event 2
In a simple carburetor, the gasoline is stored in the float bowl. A passage in the bottom of the float bowl
leads to a discharge nozzle that is located in the center of the venturi.
When the engine is running, each piston acts like an air pump during its intake stroke. A low-pressure area
(vacuum) is created in the engine cylinder, and outside air rushes through the carburetor bore to fill the low-
pressure area above the piston.
Air at atmospheric pressure is pushing down on the fuel in the float bowl. The discharge nozzle in the
venturi is placed above the level of the fuel in the bowl. When the air rushing through the venturi creates a
low-pressure area at the tip of the fuel nozzle, the atmospheric pressure on the fuel in the bowl will push
fuel out through the tip of the nozzle. Gasoline leaving the nozzle and entering the airstream is broken into
small droplets (atomized) by the fast-moving air.
The amount of air that is mixed with each part of fuel is called the air-fuel ratio. The air-fuel ratio is the
weight of one part of a mixture compared to the other part in pounds. For example, 15 pounds of air mixed
with 1 pound of fuel is an air-fuel ratio of 15:1. The mixture is made richer by adding more fuel or
removing some air.
The air-fuel ratio for an average engine operating at normal cruising speed is about 15:1. When more power
is needed or when the engine is at an idle, a richer mixture, such as 13:1 or 12:1, is required.