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 Check 

Fuel pressure and oil pressure must be within the established tolerance (green arc) for the engine. 

## Propeller Pitch Check 

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. 

## Power Check 

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 

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: 

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. 

## Idle Speed and Idle Mixture Checks 

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 

