High Speed System.

TM 10-3930-644-14&P

As the throttle valve is slowly opened from the

the venturi. At the same time, the fuel that is

slow idle position it gradually subjects the

stored in the accelerating well is also forced

secondary idle orifice to intake manifold vacuum,

through the nozzle air bleeds into the nozzle.

and the secondary idle orifice no longer bleeds

Because of the size of the power jet, the fuel in

air to the idle fuel passage but feeds an

the accelerating well will soon be exhausted and

additional quantity of fuel into the engine. This

air will then enter through the nozzle air bleeds

is proper since the throttle valve is now open

to mix with the fuel passing through the nozzle.

wider and will admit a greater amount of air to

The amount of air that can enter into the nozzle

blend with this additional fuel to maintain the

is limited by the size of the nozzle air vent.

correct proportions of fuel and air for the engine.

The result of air bleeding into the nozzle is, to

As the throttle valve is opened still wider, the idle

help atomize or break up the fuel into finer

fuel delivery begins to fade out, however, the

particles, to regulate the quantity and the rate of

throttle valve at this point is far enough open for

discharge of the fuel fed from the accelerating

the power fuel feed system to begin functioning.

well, during acceleration, and to provide the

correct mixture proportions for full throttle

3. High Speed System.

operation.

With the throttle valve (Fig. 4-3) in slow or just

As the throttle valve is opened toward the wide

off slow idle position, fuel rises up through the

open position the velocity through the venturi

nozzle and out the nozzle air bleeds to fill the

continues to increase, lowering the air pressure

accelerating well to approximately the height of

at the nozzle and resulting in additional fuel

the fuel level in the fuel bowl.

being supplied to the engine as the speed is

increased.

When the throttle valve is opened suddenly from

slow or just off slow idle position, the fuel stored

in the accelerating well is forced out through the

nozzle air bleeds very rapidly and serves to

provide the extra richness required by the

engine to meet the sudden load. When the

throttle valve is closed fuel again fills the

accelerating well ready for the next acceleration.

4. Back Suction System.

The amount of fuel supplied to an engine is

controlled by the size of the power jet, and the

difference in air pressure between the fuel bowl

chamber and the venturi. However, in many

engines the mixture must be leaned out

additionally during part throttle operation to

obtain maximum economy.

To provide this

leaner mixture a Back Suction Economizer

System is used. With this method of metering

Figure 4-3. High Speed System

fuel, the air pressure in the fuel bowl chamber is

regulated and controlled according to load

As the engine speed is increased from the slow

conditions by a combination of bowl vent and

idle position the air flow through the venturi is

economizer passages communicating with the

gradually increased, the velocity through the

throttle bore of the carburetor.

Through

venturi is high enough to create a pressure at

regulations of the air pressure in the fuel bowl

the tip of the nozzle slightly less than the

chamber

pressure in the fuel bowl chamber and the

accelerating well. Fuel, therefore, feeds from

the fuel bowl through the power jet and out the

nozzle to be discharged into the air stream at

R-123-1

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