CARBURETOR

A carburetor is basically a device for mixing air and fuel in the correct amounts for efficient combustion. The carburetor bolts to the engine intake manifold. The air cleaner fits over the top of the carburetor to trap dust and dirt. The basic carburetor consists of the following parts:

• Carburetor body. The carburetor body is a cast metal housing for the carburetor components.
• Usually the main body houses the fuel bowl, main jets, air bleeds, power valve, pump checks, diaphragm type accelerator pump, venturis, circuit passages, and float mechanism. The body is flanged on the bottom to allow the carburetor to be bolted to the intake manifold.
• Air horn. The air horn is also called the throat or barrel. It routes outside air into the engine intake manifold. It contains the throttle valve, the venturi, and the outlet end of the main discharge tube. The parts which often fasten to the air horn body are as follows: the choke, the hot idle compensator, the fast idle linkage rod, the choke vacuum break, and sometimes the float and pump mechanisms.
• Throttle valve (fig. 4-19). This disc-shaped valve controls air flow through the air horn.
• When closed, it restricts the flow of air and fuel into the engine, and when opened, air flow, fuel flow, andengine power increase.
• Venturi. The venturi produces sufficient suction to pull fuel out of the main discharge tube.
• Main discharge tube. The main discharge tube is also called the main fuel nozzle. It uses venturi vacuum to feed fuel into the air horn and engine. It is a passage that connects the fuel bowl to the center of the venturi.
• Fuel bowl. The fuel bowl holds a supply of fuel that is NOT under fuel pump pressure.

Carburetor size is stated in CFM (cubic feet of air per minute). This is the amount of air that can flow through the carburetor at wide, open throttle. CPM is an indication of the maximum air flow capacity. Usually, small CPM carburetors are more fuel-efficient than larger carburetors. Air velocity, fuel mixing, and atomization are better with small throttle bores. A larger CPM rating is desirable for high engine power output.

A carburetor system or circuit is a network of passages and related parts that help control the air-fuel ratio under specific engine-operating conditions. The seven basic carburetor systems are the following:

• Float system
• Idle system
• Off idle system
• Acceleration system
• High-speed system
• Full-power system
• Chore system

Understanding each of these systems is important. It will help you when diagnosing and repairing carburetor problems.