Carburetor

TM 10-3930-633-12

4-56. Carburetor

(2) Adjust the idle mixture screw to obtain

the highest RPM. While making the adjustment,

a. General. The carburetor consists of three

carefully watch the tachometer and notice that

main assemblies; the air intake body, main body

the speed can be decreased by turning the screw

and the throttle body. The air intake body, which

in either direction from the setting that gave the

serves as the main body cover, contains the choke

highest RPM reading.

plate, vent tube, and float and lever assembly.

(3) From the highest idle speed setting, turn

The anti-stall dashpot is bracket-mounted to the

the mixture screw clockwise (leaner) until the

air intake body. The main body contains the

speed starts to drop. Turn the screw in the Op -

accelerator pump assembly, metering rod jet,

posite direction (counterclockwise) just far

low-speed jet, accelerator pump check ball and

enough to recover the speed that was lost.

weight, an anti-percolator bleed and the main

discharge nozzle. The throttle body contains the

NOTE

This procedure will assure that the idle has been

throttle plate, throttle shaft and lever and idle

set to the leanest mixture possible for smooth idle.

mixture adjusting screw with a plastic limiter cap

This setting is very important.

and choke connector rod (manuaI choke car-

(4) Since the correct speed was originally set

buretors).

using the speed screw, the speed obtained after

b. Operation. The engine speed is regulated

finding the leanest smooth idle setting will

and controlled by a proportion of fuel and air

probably be too fast.

delivered to the cylinders for all engine operating

(5) Readjust the speed screw to obtain

conditions. Operation is based on the principle of

correct idle speed. Repeat steps (2) and (3) above.

pressure differences. Air is drawn into the car-

buretor air horn by mainfold vacuum. As the air

passes through the carburetor on the way to enter

the cylinders, low pressure is created at the fuel

discharge outlets of the carburetor. The fuel bowl

is vented to the air intake body. The high air

pressure exerted on the fuel in the bowl forces the

fuel to travel up through the fuel discharge

channels and out into the air stream passing

through the carburetor. The fuel and air is mixed

at the point and distributed into the engine

cylinders for combustion.

4-57. Carburetor Adjustment

Before attempting carburetor adjustment, check

the condition of the following:

a. Spark Plugs. See that plugs are correct

type, clean, and have correct gap.

b. Distributor Points. See that points are

clean, in good condition and properly set.

c. All High Tension Terminals. See t h a t

terminals are making good contact at plugs and

at distributor cap and the coil towers.

d. Carburetor. Check that carburetor is

securely mounted to mainfold, and that no fuel or

air leakage is evident.

e. Adjustment. Before making idle speed

Figure 4-29. Carburetor adjustments

adjustment, have the engine warmed up. For best

4-58. Carburetor Replacement

results, use a tachometer to determine engine

speed during these adjustments. Refer to figure 4-

a. General. Flooding, stumble on acceleration

29 and proceed as follows:

and other performance complaints are, in many

instances, caused by the presence of dirt, water,

(1) Turn the idle speed screw in or out to

obtain 500 RPM reading on the tachometer. Be

or other foreign matter in the carburetor. To aid

sure that the choke valve is filly open and that

in diagnosing the complaint, the carburetor

the fast idle adjusting screw is not contacting the

should be carefully removed from the engine

cam.

without removing the fuel from the bowl. The

4-44