TOPIC 4a. CARBURETOR (SRT)

TM 10-3930-644-14&P

TOPIC 4a. CARBURETOR (SRT)

A. DESCRIPTION

2. Idle System. At idle speed the throttle plate is

advanced slightly to expose the upper idle

The carburetor is of the single barrel updraft design, with

discharge hole to engine manifold vacuum

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

(suction), see Figure 4.

This suction is

bowl to permit operation at quite extreme angles without

transmitted to the idle jet through a passage

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

connecting the idle discharge holes with the idle

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

jet. Fuel for idle is supplied through the main jet

and idle operation must enter through the air cleaner.

to a well at the bottom of the discharge jet. The

The fuel supply system is made up of the threaded fuel

fuel for idle flows out of this well through a

inlet, fuel valve (needle and seat), float assembly and

restricted drilling at the bottom of the idle fuel

the float chamber. The idle system consists of two Idle

pick-up passage. From here the fuel is metered

discharge holes, idle air passage, idle adjusting needle,

through the idle jet calibration before entering

idle jet, and fuel pick up passage. The high speed (main

the vacuum passage leading to the idle

metering) system consists of the venturi, main jet, main

discharge holes. As the fuel leaves the idle Jet

discharge and well vent. Some models also include a

it is mixed with air that originates back of (or

main jet adjustment. The choke system is of the semi-

from behind) the venturi. The position of the idle

automatic type and is made up of a choke plate, with a

adjusting needle in this passage controls the

spring loaded poppet valve, mounted on a shaft located

suction on the idle jet and thereby the idle fuel

within the air intake and operated externally by a lever

air mixture. Turning the idle adjusting needle IN,

attached to the choke shaft.

(clockwise) results in a greater suction on the

idle jet with a smaller amount of air admitted to

B. OPERATION

give a richer mixture. Turning the needle OUT

(counterclockwise) increases the amount of idle

1. Fuel Supply System. Fuel under pressure is

air admitted and reduces the suction on the idle

supplied through the fuel inlet fitting, fuel valve

jet resulting in a leaner mixture. This idle fuel-air

(needle and seat) to the float chamber, see

mixture is then discharged through the idle

Figure 3.

The float in the float chamber

discharge holes into the air stream.

automatically regulates the opening through the

fuel valve (needle and seat) to maintain the

proper level of fuel in the fuel bowl and to meet

the demands of the engine according to engine

load and speed.

Figure 4. Idle System

Figure 3. Fuel Supply System

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