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 valve, opens into the float chamber. When the valve is completely closed, pressures on the fuel in the float chamber and at the discharge nozzle are almost equal, and fuel flow is reduced to maximum lean. With the valve wide open, pressure on the fuel in the float chamber is greatest and fuel mixture is richest. Adjusting the valve to positions between these two extremes controls the mixture. The quadrant in the cockpit is usually marked “lean” near the back end and “rich” at the forward end. The extreme back position is marked “idle cutoff” and is used when stopping the engine.
On float carburetors equipped with needle-type mixture control, placing the mixture control in idle cutoff seats the needle valve, thus shutting off fuel flow completely. On carburetors equipped with back-suction mixture controls, a separate idle cutoff line, leading to the extreme low pressure on the engine side of the throttle valve, is incorporated. (See the dotted line in Figure 2-17.) The mixture control is so linked that when it is placed in the “idle cutoff” position, it opens another passage that leads to piston suction. When placed in other positions, the valve opens a passage leading to the atmosphere. To stop the engine with such a system, close the throttle and place the mixture in the “idle cutoff” position. Leave the throttle until the engine has stopped running and then open the throttle completely.