When ignition in the cylinder occurs before the optimum  crankshaft position is reached, the timing is said to be early.  If ignition occurs too early, the piston rising in the cylinder  is opposed by the full force of combustion. This condition  results in a loss of engine power, overheating, and possible  detonation and preignition.  If ignition occurs at a time after the optimum crankshaft  position is reached, the ignition timing is said to be late. If it  occurs too late, not enough time is allowed to consume the  fuel-air charge, and combustion is incomplete. As a result,  the engine loses power and requires a greater throttle opening  to carry a given propeller load. Moisture forming on different parts of the ignition system  causes more common irregularities. Moisture can enter  ignition system units through cracks or loose covers, or it  can result from condensation. Breathing, a situation that  occurs during the readjustment of the system from low to  high atmospheric pressure, can result in drawing in moisture- laden air. Ordinarily, the heat of the engine is sufficient  to evaporate this moisture, but occasionally the moist air  condenses as the engine cools. The result is an appreciable  moisture accumulation which causes the insulation materials  to lose electrical resistance. A slight amount of moisture  contamination may cause reduction in magneto output by  short-circuiting to ground a part of the high-voltage current  intended for the spark plug. If the moisture accumulation is  appreciable, the entire magneto output may be dissipated to  ground by way of flashover and carbon tracking. Moisture  accumulation during flight is extremely rare because the high  operating temperature of the system is effective in preventing  condensation. Difficulties from moisture accumulation are  probably more evident during starting and ground operation. Spark plugs are often diagnosed as being faulty when the real  malfunction exists in a different system. Malfunctioning of  the carburetor, poor fuel distribution, too much valve overlap,  leaking primer system, or poor idle speed and mixture settings  show symptoms that are the same as those for faulty ignition.  Unfortunately, many of these conditions can be temporarily  improved by a spark plug change, but the trouble recurs in a  short time because the real cause of the malfunction has not  been eliminated. A thorough understanding of the various  engine systems, along with meticulous inspection and good  maintenance methods, can substantially reduce such errors. Most reciprocating engines have timing reference marks  built into the engine. The timing reference marks vary by  manufacturer.  [Figure 4-36]  When the starter gear hub is  installed correctly, the timing marks are marked on it that  line up with the mark on the starter. On an engine that has no  starter gear hub, the timing mark is normally on the propeller  flange edge.  [Figure 4-37]  The top center (TC) mark stamped  on the edge aligns with the crankcase split line below the  crankshaft when the No. 1 piston is at top dead center. Other  flange marks indicate degrees before top center.  Some engines have degree markings on the propeller  reduction drive gear. To time these engines, the plug  provided on the exterior of the reduction gear housing must  be removed to view the timing marks. On other engines, the  timing marks are on a crankshaft flange and can be viewed  Figure 4-36.  Lycoming timing marks. Figure 4-37.  Propeller flange timing marks.  Magneto-Ignition Timing Devices Built-In Engine Timing Reference Marks 4-23 T C 24 32 T C 24 32 Timing marks Split line Index mark is a  dot on the starter Number of degrees of crank- shaft rotation is marked on  the propeller hub