engine rpm specified for the ignition system check, allow the  rpm to stabilize. Place the ignition switch in the right position  and note the rpm drop on the tachometer. Return the switch  to the both position. Allow the switch to remain in the both  position for a few seconds so that the rpm stabilizes again.  Place the ignition switch in the left position and again note the  rpm drop. Return the ignition switch to the both position. Note  the amount of total rpm drop that occurs for each magneto  position. The magneto drop should be even for both magnetos  and is generally in the area of a 25–75 rpm drop for each  magneto. Always refer to the aircraft operating manual for  specific information. This rpm drop is because operating on  one magneto combustion is not as efficient as it is with two  magnetos providing sparks in the cylinder.  death. If the propeller is turned in this condition, the engine  can start with personnel in the propeller arch. Inspection of ignition leads should include both a visual  and an electrical test. During the visual test, the lead cover  should be inspected for cracks or other damage, abrasions,  mutilated braid, or other physical damage. Inspect leads for  overheating if routed close to exhaust stacks. Disconnect the  harness coupling nuts from the top of the spark plugs and  remove the leads from the spark plug lead well. Inspect the  contact springs and compression springs for any damage  or distortion and the sleeves for cracks or carbon tracking.  The coupling nut that connects to the spark plug should be  inspected for damaged threads or other defects.  Remember, this tests not only the magnetos but also the  ignition leads and spark plugs. If either magneto has  excessive rpm drop while operating by itself, the ignition  system needs to be checked for problems. If only one  magneto has a high magneto drop, the problem can be  isolated and corrected by operating on that magneto. This  ignition system check is usually performed at the beginning  of the engine run-up because rpm drops not within the  prescribed limits affect later checks. Each lead should be checked for continuity using a high- tension lead tester by connecting the black lead to the contact  spring and the red lead to the eyelet of the same lead in the  cover. The continuity lamp on the tester should illuminate  when tested. The insulation resistance test of each lead is  accomplished using the high-tension lead tester by attaching  the red, or high-voltage, lead to the spring of the harness  lead. Then, attach the black lead to the ferrule of the same  lead. Depress the press-to-test push button switch on the lead  tester. Observe that the indictor lamp flashes and gap fires  simultaneously as long as the press-to-test switch is held in  the depressed position.  The ignition switch check is performed to see that all  magneto ground leads are electrically grounded. The  ignition switch check is usually made at 700 rpm. On those  aircraft engine installations that do not idle at this low rpm,  set the engine speed to the lowest possible to perform this  check. When the speed to perform this check is obtained,  momentarily turn the ignition switch to the off position.  The engine should completely quit firing. After a drop of  200–300 rpm is observed, return the switch to the both  position as rapidly as possible. Do this quickly to eliminate  the possibility of afterfire and backfire when the ignition  switch is returned to both. If the indicator lamp flashes and the gap fails to fire, the lead  under test is defective and must be replaced. The indicator  lamp flashes to show that a high-voltage impulse was sent  out. If it fails to pass through the tester, then the electrical  pulse leaked through the wire showing it to be defective.  When defective leads are revealed by an ignition harness test,  continue the test to determine whether the leads or distributor  block are defective. If the difficulty is in an individual ignition  lead, determine whether the electrical leak is at the spark plug  elbow or elsewhere. Remove the elbow, pull the ignition lead  out of the manifold a slight amount, and repeat the harness  test on the defective lead. If this stops the leakage, cut away  the defective portion of the lead and reinstall the elbow  assembly, integral seal, and terminal (sometimes referred to  as cigarette).  [Figure 4-47] If the ignition switch is not returned quickly enough, the  engine rpm drops off completely and the engine stops. In  this case, leave the ignition switch in the off position and  place the mixture control in the idle-cutoff position to avoid  overloading the cylinders and exhaust system with raw fuel.  When the engine has completely stopped, allow it to remain  inoperative for a short time before restarting. If the lead is too short to repair in the manner described, or the  electrical leak is inside the harness, replace the defective lead.  Single ignition lead replacement procedures are as follows: If the engine does not cease firing in the off position, the  magneto ground lead, more commonly referred to as the P  lead, is open, and the trouble must be corrected. This means  that one or more of the magnetos are not being shut off even  when the ignition switch is in the off position. Turning the  propeller of this engine can result in personnel injury or  1.  Disassemble the magneto or distributor so that the  distributor block is accessible. 4-28 Maintenance and Inspection of Ignition Leads Ignition Switch Check