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. that is closed at throttle settings below approximately 60–70  percent of rated power. This system, like the accelerating  system, is operated by the throttle control. When the throttle valve is opened quickly, a large volume  of air rushes through the air passage of the carburetor; the  amount of fuel that is mixed with the air is less than normal  due to the slow response rate of the main metering system.  As a result, after a quick opening of the throttle, the fuel/air  mixture leans out momentarily. This can cause the engine to  accelerate slowly or stumble as it tries to accelerate.  A typical economizer system consists of a needle valve  which begins to open when the throttle valve reaches  a predetermined point near the wide-open position.  [Figure 2-20]  As the throttle continues to open, the needle  valve is opened further and additional fuel flows through  it. This additional fuel supplements the flow from the main  metering jet direct to the main discharge nozzle. A pressure-operated economizer system is shown in  Figure 2-21 . This type has a sealed bellows located in an  enclosed compartment. The compartment is vented to engine  To overcome this tendency, the carburetor is equipped with a  small fuel pump called an accelerating pump. A common type  of accelerating system used in float carburetors is illustrated  in  Figure 2-18 . It consists of a simple piston pump operated  through linkage by the throttle control and a passageway  opening into the main metering system or the carburetor  barrel near the venturi. When the throttle is closed, the piston  moves back, and fuel fills the cylinder. If the piston is pushed  forward slowly, the fuel seeps past it back into the float  chamber; if pushed rapidly, it sprays fuel in the venturi and enriches the mixture. An example of a cutaway accelerator  pump is shown in  Figure 2-19 . For an engine to develop maximum power at full throttle, the  fuel mixture must be richer than for cruise. The additional  fuel is used for cooling the engine combustion chambers to  prevent detonation. An economizer is essentially a valve  Figure 2-19.  Accelerating pump shown in cutaway. Economizer System 2-14 Accelerating System  Figure 2-18.  Accelerating system.  Economizer opening Force downward