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out the idling jet. A separate air bleed, known as the idle air bleed, is included as part of the idling system. It functions in the same manner as the main air bleed. An idle mixture adjusting device is also incorporated. A typical idling system is illustrated in Figure 2-15 . 

## Mixture Control System 

As altitude increases, the air becomes less dense. At an altitude of 18,000 feet, the air is only half as dense as it is at sea level. This means that a cubic foot of space contains only half as much air at 18,000 feet as at sea level. An engine cylinder full of air at 18,000 feet contains only half as much oxygen as a cylinder full of air at sea level. 

The low pressure area created by the venturi is dependent upon air velocity rather than air density. The action of the venturi draws the same volume of fuel through the discharge nozzle at a high altitude as it does at a low altitude. Therefore, the fuel mixture becomes richer as altitude increases. This can be overcome either by a manual or an automatic mixture control. 

On float-type carburetors, two types of purely manual or cockpit controllable devices are in general use for controlling fuel/air mixtures, the needle type and the back-suction type. [Figures 2-16 and  2-17] 

With the needle-type system, manual control is provided by a needle valve in the base of the float chamber. 

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[Figure 2-16] This can be raised or lowered by adjusting a control in the cockpit. Moving the control to “rich,” opens the needle valve wide, which permits the fuel to flow unrestricted to the nozzle. Moving the control to “lean,” partially closes the valve and restricts the flow of fuel to the nozzle. 

The back-suction-type mixture control system is the most widely used. [Figure 2-17] In this system, a certain amount of venturi low pressure acts upon the fuel in the float chamber so that it opposes the low pressure existing at the main discharge nozzle. An atmospheric line, incorporating an adjustable 

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