Low  High Airflow in lb/hr High Fuel/air mixture Low Normal F/A mixture auto-rich position Dry takeoff power F/A mixture with water injection Wet takeoff power Low  High Airflow in lb/hr Rich Fuel/air Lean Idle Climb Minimum cruise Takeoff Maximum cru ise Rich Manual lean Low  High Airflow in lb/hr Rich Fuel/air Lean Idle Rated pow er Minimum cruise Takeoff power Maximum cru ise Figure 10-47.  Typical fuel-air mixture curve for a float-type  carburetor.  Figure 10-48.  Idle mixture curve.  Figure 10-46.  Fuel/air curve for a water-injection engine.  solely for cooling. A leaner mixture would give more power.  Actually, water or, more accurately, the antidetonant (water/ alcohol) mixture is a better coolant than extra fuel. Therefore,  water injection permits higher manifold pressures and a still  further increase in power. In establishing the final curve for engine operation, the  engine’s ability to cool itself at various power settings is, of  course, taken into account. Sometimes the mixture must be  altered for a given installation to compensate for the effect  of cowl design, cooling airflow, or other factors on engine  cooling. The final fuel/air mixture curves take into account  economy, power, engine cooling, idling characteristics, and  all other factors that affect combustion. Figure 10-47  shows a typical final curve for a float-type  carburetor. Note that the fuel/air mixture at idle is the same  in rich and in manual lean. The mixture remains the same  until the low cruise range is reached. At this point, the curves  separate and then remain parallel through the cruise and  power ranges. Note the spread between the rich and lean setting in the  cruise range of both curves. Because of this spread, there  is a decrease in power when the mixture control is moved  from auto-rich to auto-lean with the engine operating in the  cruise range. This is true because the auto-rich setting in  the cruise range is very near the best power mixture ratio.  Therefore, any leaning out gives a mixture that is leaner  than best power. The idle mixture curve shows how the mixture changes when  the idle mixture adjustment is changed.  [Figure 10-48]  Note  that the greatest effect is at idling speeds. However, there  is some effect on the mixture at airflows above idling. The  airflow at which the idle adjustment effect cancels out varies  from minimum cruise to maximum cruise. The exact point  depends on the type of carburetor and the carburetor setting.  In general, the idle adjustment affects the fuel/air mixture up  to low cruise on engines equipped with float-type carburetors.  This means that incorrect idle mixture adjustments can easily  give faulty cruise performance, as well as poor idling.  There are variations in mixture requirements between  one engine and another because of the fuel distribution  within the engine and the ability of the engine to cool.  Remember, a carburetor setting must be rich enough to  supply a combustible mixture for the leanest cylinder. If fuel  distribution is poor, the overall mixture must be richer than  would be required for the same engine if distribution were  good. The engine’s ability to cool depends on such factors  as cylinder design (including the design of the cooling fins),  Idle Mixture 10-33