1 2 3 4 5 6 7 8 9  10 11  12 13 14 Lengthened Valve overlap as affected by valve clearance Reduced Lean Effect on fuel/air mixture or combustible  charge Rich Figure 10-51.  Effect of variation in valve overlap on fuel/air mixture between cylinders.  The need for troubleshooting normally is dictated by poor  operation of the complete powerplant. Power settings for the  type of operation at which any difficulty is encountered, in  many cases, indicate that part of the powerplant that is the  basic cause of difficulty. With proper precautions, manifold pressure can be taken as a  measure of power input, and rpm can be taken as a measure  of power output. However, the following factors must  be considered: The cylinders of an engine, along with any type of  supercharging, form an air pump. Furthermore, the power  developed in the cylinders varies directly with the rate that  air can be consumed by the engine. Therefore, a measure  of air consumption or airflow into the engine is a measure  of power input. Ignoring for the moment such factors as  humidity and exhaust back pressure, the manifold pressure  gauge and the engine tachometer provide a measure of  engine air consumption. Thus, for a given rpm, any change  in power input is reflected by a corresponding change in  manifold pressure. The power output of an engine is the power absorbed by the  propeller. Therefore, propeller load is a measure of power  output. Propeller load, in turn, depends on the propeller  rpm, blade angle, and air density. For a given angle and air  density, propeller load (power output) is directly proportional  to engine speed. The basic power of an engine is related to  manifold pressure, fuel flow, and rpm. Because the rpm of  the engine and the throttle opening directly control manifold  pressure, the primary engine power controls are the throttle  and the rpm control. An engine equipped with a fixed-pitch  propeller has only a throttle control. In this case, the throttle  setting controls both manifold pressure and engine rpm. Having relative measures of power input and power output,  the condition of an engine can be determined by comparing  input and output. This is done by comparing the manifold  pressure required to produce a given rpm with the manifold  pressure required to produce the same rpm at a time when  the engine (or an identical powerplant) was known to be in  top operating condition. 1.  Atmospheric pressure and air temperature must be  considered, since they affect air density. 2.  These measures of power input and power output  should be used only for comparing the performance  of an engine with its previous performance, or for  comparing identical powerplants. 3.  With a controllable propeller, the blades must be  against their low-pitch stops, since this is the only  blade position in which the blade angle is known and  does not vary. Once the blades are off their low-pitch  stops, the propeller governor takes over and maintains  a constant rpm, regardless of power input or engine  condition. This precaution means that the propeller  control must be set to maximum or takeoff rpm, and  the checks made at engine speeds below this setting. 10-36