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 . 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.  [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  Figure 2-17. Back-suction-type mixture control system. Figure 2-15.  Idling system.  Figure 2-16.  Needle-type mixture control system. Mixture Control System 2-13 Full rich position Back suction line Idle cutoff Idle cutoff line 14 p.s.i. Idle mixture adjustment Idling jet Idle air bleed 13.5 p.s.i. 11 p.s.i. M Full rich