Figure 4-4. Back Suction System

TM 10-3930-644-14&P

As this vacuum decreases as the throttle

approaches wide open position, less air is drawn

out of the fuel bowl chamber and additional fuel

flows to the engine to provide the extra richness

required for operation at heavy loads where

maximum horsepower is necessary.

5. Choke System.

The choke system is used during cold starting

and the warm-up period. Under these cold

conditions it is necessary to supply an extra rich

mixture of fuel and air, as only the "light ends" or

more volatile portions of the fuel will vaporize

with the manifold and air temperatures at the

Figure 4-4. Back Suction System

cold temperatures. Consequently it is necessary

that a large quantity of fuel be available so that

the fuel flow through the carburetor can be

there will be enough "light ends," to combine

controlled to provide the proper mixture

with the air to form a combustible mixture for

proportions for the engine.

starting the engine.

All the air that enters the fuel bowl chamber (Fig.

The function of the choke valve (Fig. 4-5) is to

4-4) must first pass through the air cleaner and

restrict the amount of air that can enter the

the bowl vent. The size of the bowl vent controls

carburetor and to increase the vacuum on the

or limits the amount of air that can enter the fuel

nozzle so that additional fuel will be drawn into

bowl chamber. The amount of air that is drawn

the manifold. As soon as the engine fires and

out of the fuel bowl chamber is controlled by the

runs the rich mixture must be rapidly reduced to

size of the economizer jet, the economizer

prevent stalling. This change in mixture should

orifice and the position of the throttle valve as its

be accomplished by the operator repositioning

position determines the manifold vacuum or

the choke valve. To help reduce the sensitivity

suction on the economizer orifice.

As the

of the choke valve position, use is made of a

throttle valve is opened from the fast idle

spring loaded relief valve. This valve opens

position the economizer orifice is gradually

automatically with engine speed and load and

exposed to manifold suction, and air flows from

eliminates a great deal of manipulation of the

the fuel bowl chamber, through the economizer

choke on the part of the operator.

jet and out the economizer orifice. This air must

be replaced by air entering through the bowl

vent but as the size of the bowl vent restricts the

amount of air that can enter, the resultant.

Pressure in the fuel bowl chamber will be

lowered, reducing the difference in air pressure

between the nozzle and the fuel bowl chamber.

The flow of fuel will therefore be retarded so that

the exact economy mixture ratio will be delivered

to the engine at this particular throttle opening.

Opening the throttle valve further exposes the

entire economizer orifice to manifold suction,

resulting in additional air being removed from

the fuel bowl chamber, again leaning out the

mixture ratio to the correct proportions for this

new throttle position.

After the economizer

orifice is fully exposed to manifold vacuum, the

amount of air that is drawn out of the fuel bowl

chamber is controlled by the manifold vacuum or

Figure 4-5. Choke System

(or suction) at any given throttle valve position.

3-61