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
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
VENTURI. The venturi produces sufficient
suction to pull fuel out of the main discharge tube.
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:
1. FLOAT SYSTEM
2. IDLE SYSTEM
3. OFF IDLE SYSTEM
4. ACCELERATION SYSTEM
5. HIGH-SPEED SYSTEM
6. FULL-POWER SYSTEM
7. CHORE SYSTEM
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
Figure 4-19.Simple carburetor with throttle valve.