In the simple carburetor described, the speeding-up of the engine will result in a greater rush of air through the intake pipe, which in turn will suck out a much greater quantity of fuel. If the carburetor is adjusted to give the proper quantity of fuel for the air that passes at low speed, at high speed it will give far more fuel than will be required by the quantity of air that then passes. Thus at high speed the mixture will be too rich. If, on the other hand, this carburetor is adjusted to give a proper mixture at high speed, too little fuel will be sucked out when the engine runs slowly, and the mixture will be too lean. A carburetor must thus have an additional device that will keep the mixture correct, regardless of the speed at which the engine runs. This is sometimes done by changing the size of the spray nozzle so that a greater or less quantity of fuel flows out, but more usually by permitting an extra quantity of air to enter the carburetor as the engine speeds up. This is done with an _extra air intake_, the principle of which is illustrated in Figure 22. As will be seen, this carburetor has two openings for air, one being the main air inlet and the other the extra air inlet. The latter is an opening provided with a valve which is held on its seat by a spring. The suction created by an inlet stroke is exerted in the carburetor, but at low speed is not sufficient to suck the extra air valve from its seat. Air then enters only through the main air inlet, and the spray nozzle is adjusted to give the proper proportion of fuel. [Illustration: FIG. 22.—PRINCIPLE OF EXTRA AIR INLET] As the engine speed increases the mixture becomes richer; but there is also an increase in suction, which becomes strong enough to pull the extra air valve from its seat. This provides another opening into the carburetor, through which enough air enters to keep the mixture in proper proportion. The higher the speed of the engine the more the valve will open, and the greater will be the quantity of air admitted. In order to get the fullest power from an engine, the carburetor is built to give its most perfect mixture at the usual working speed. This will be the speed at which the engine will run under ordinary conditions. As the engine will run at this speed most of the time, the carburetor should then deliver its best mixture on the least possible quantity of fuel. As an engine is run at low speed so little of the time, it is not necessary that the mixture should then be so perfect or that the fuel should be used so economically.