confining maintenance to the specific part of the ignition  system that is responsible for the trouble. Fast rpm drop is usually the result of either faulty spark  plugs or faulty ignition harness. This is true because faulty  plugs or leads, take effect at once. The cylinder goes dead  or starts firing intermittently the instant the switch is moved  from “both” to the “right” or “left” position. Slow rpm drop usually is caused by incorrect ignition timing  or faulty valve adjustment. With late ignition timing, the  charge is fired too late (in relation to piston travel) for the  combustion pressures to build up to the maximum at the  proper time. The result is a power loss greater than normal for  single ignition because of the lower peak pressures obtained  in the cylinder. However, this power loss does not occur as  rapidly as that which accompanies a dead spark plug. This  explains the slow rpm drop as compared to the instantaneous  drop with a dead plug or defective lead. Incorrect valve  clearances, through their effect on valve overlap, can cause  the mixture to be too rich or too lean. The too rich or too  lean mixture may affect one plug more than another, because  of the plug location and show up as a slow rpm drop on the  ignition check. Switch from “both” to “right” and return to  “both.” Switch from “both” to “left” and return to “both.”  Observe the rpm drop while operating on the right and  left positions. The maximum drop should not exceed that  specified by the engine manufacturer. Fuel pressure and oil pressure must be within the established  tolerance (green arc) for the engine. The propeller is checked to ensure proper operation of the  pitch control and the pitch-change mechanism. The operation  of a controllable pitch propeller is checked by the indications  of the tachometer and manifold pressure gauge when the  propeller governor control is moved from one position to  another. Because each type of propeller requires a different  procedure, the applicable manufacturer’s instructions should  be followed. Specific rpm and manifold pressure relationship should be  checked during each ground check. This can be done at the  time the engine is run-up to make the magneto check. The  purpose of this check is to measure the performance of the  engine against an established standard. Calibration tests have  determined that the engine is capable of delivering a given  power at a given rpm and manifold pressure. The original  calibration, or measurement of power, is made by means of a  dynamometer in a test cell. During the ground check, power  Fuel Pressure and Oil Pressure Check Propeller Pitch Check Power Check  Idle Speed and Idle Mixture Checks is measured with the propeller. With constant conditions of  air density, the propeller, at any fixed-pitch position, always  requires the same rpm to absorb the same horsepower from  the engine. This characteristic is used in determining the  condition of the engine. With the governor control set for full low pitch, the propeller  operates as a fixed-pitch propeller, because the engine is  static. Under these conditions, the manifold pressure for any  specific engine, with the mixture control in rich, indicates  whether all the cylinders are operating properly. With one  or more dead or intermittently firing cylinders, the operating  cylinders must provide more power for a given rpm.  Consequently, the carburetor throttle must be opened further,  resulting in higher manifold pressure. Different engines of  the same model using the same propeller installation, and at  the same barometer and temperature readings, should require  the same manifold pressure to within 1 "Hg. A higher than  normal manifold pressure usually indicates a dead cylinder  or late ignition timing. An excessively low manifold pressure  for a particular rpm usually indicates that the ignition timing  is early. Early ignition can cause detonation and loss of power  at takeoff power settings. The accuracy of the power check may be affected by the  following variables: Plug fouling difficulty is the inevitable result of failure to  provide a proper idle mixture setting. The tendency seems  to be to adjust the idle mixture on the extremely rich side  and to compensate for this by adjusting the throttle stop to  a relatively high rpm for minimum idling. With a properly  1.  Wind—any appreciable air movement (5 mph or more)  changes the air load on the propeller blade when it is  in the fixed-pitch position. A head wind increases the  rpm obtainable with a given manifold pressure. A tail  wind decreases the rpm. 2.  Atmospheric temperatures—the effects of variations  in atmospheric temperature tend to cancel each other.  Higher carburetor intake and cylinder temperatures  tend to lower the rpm, but the propeller load is  lightened because of the less dense air. 3.  Engine and induction system temperature—if the  cylinder and carburetor temperatures are high because  of factors other than atmospheric temperature, a low  rpm results since the power is lowered without a  compensating lowering of the propeller load. 4.  Oil temperature—cold oil tends to hold down the rpm,  since the higher viscosity results in increased friction  horsepower losses. 10-28