TOPIC 4. CARBURETOR (TYPE II) (PRT)

TM 10-3930-644-14&P

TOPIC 4. CARBURETOR(TYPE II) (PRT)

A. DESCRIPTION

The carburetor is of the single barrel updraft design, with

a single venturi, two floats, and a semi-concentric fuel

bowl to permit operation at quite extreme angles without

flooding or starving the engine. It is of the "balanced"

and "sealed" type since all air for fuel bowl ventilation

and idle operation must enter through the air cleaner.

The fuel supply system is made up of the threaded fuel

inlet, and the float chamber. The idle system consists of

two idle discharge holes, idle air passage, idle adjusting

needle, idle jet, and fuel pickup passage. The high

speed (main metering) system consists of the venturi,

main jet, main discharge and well vent. The choke

system is of the semi-automatic type and is made up of

a choke plate, with a spring loaded poppet valve,

mounted on a shaft located within the air intake and

operated externally by a lever attached to the choke

shaft.

B. OPERATION

1. Fuel Supply System.

Figure 4-2. Idle System

Fuel under pressure is supplied through the fuel

2. Idle System.

inlet fitting, fuel valve (needle and seat) to the

float chamber, see Figure 4-1. The float in the

The idle system controls the flow of fuel at idle

float chamber automatically regulates the

speed and at slow speeds until the throttle is

opening through the fuel valve (needle and seat)

opened wide enough to allow the power fuel

to maintain the proper level of fuel in the fuel

feed system to function.

bowl and to meet the demands of the engine

according to engine load and speed.

When the throttle valve (Fig. 4-2) is in the idle

position the edge of the valve is between the

primary idle orifice and the secondary idle

orifice. With the valve in this position the air

pressure (manifold vacuum) at the primary idle

orifice is lower than the air pressure in the fuel

bowl chamber and fuel is forced from the fuel

bowl into the Idle fuel passage. As the fuel

travels through the idle fuel passage it passes

through the metering orifice of the idle jet to the

point where it is combined with air entering

through the idle adjusting needle seat. The

mixing of air with gasoline helps to atomize the

fuel and the process is repeated at the

secondary idle orifice as the fuel travels through

the idle fuel passage. As this rich mixture of fuel

and air emerges from the primary idle orifice it is

reduced to correct proportions by the air which

passes around the throttle valve since this valve

must be slightly open to permit the engine to

idle.

The resultant mixture is correct for

Figure 4-1. Fuel Supply System

operating engine at idle speed, provided the idle

adjusting needle is properly adjusted.

R-123-1

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