Check for continuity by grounding the lead at the cylinder  and then checking at the distributor block end to establish that  the lead grounded is as designated on the band for the lead. Carbon fouling from fuel is associated with mixtures that  are too rich to burn or mixtures that are so lean they cause  intermittent firing.  [Figure 4-48]  Each time a spark plug does  not fire, raw fuel and oil collect on the nonfiring electrodes  and nose insulator. These difficulties are almost invariably  associated with an improper idle mixture adjustment, a  leaking primer, or carburetor malfunctions that cause too rich  a mixture in the idle range. A rich fuel-air mixture is detected  by soot or black smoke coming from the exhaust and by an  increase in rpm when the idling fuel-air mixture is leaned  to best power. The soot that forms as a result of overly rich  idle fuel-air mixtures settles on the inside of the combustion  chamber because the heat of the engine and the turbulence  in the combustion chamber are slight. At higher engine  speeds and powers, however, the soot is swept out and does  not condense out of the charge in the combustion chamber. After checking all leads for proper identification, cut them to  the proper length for installation into the distributor block.  Before cutting the leads, however, force them back into the  manifold as far as possible to provide surplus wire in the  ignition manifold. This extra wire may be needed at a later  date in the event that chafing of a lead at the spark plug elbow  necessitates cutting a short section of wire from the spark  plug end of the harness. After cutting each lead to length,  remove approximately  3 ⁄ 8  inch of insulation from the end and  prepare the lead for insertion into the distributor block. Before  installing the lead, back out the set screw in the distributor  block far enough to permit slipping the end of the wire into  the hole without force. Insert the lead into the block and  tighten the set screw. Connect the wires in firing order (the  first cylinder to fire No. 1 location on the block, the second  in the firing order to No. 2 location, etc).  After connecting each lead, check continuity between the  lead and its distributor block electrode with continuity light  or timing light. To perform one test lead, touch the other test  lead to the proper distributor block electrode. If the light does  not indicate a complete circuit, the set screw is not making  contact with the ignition wire or the lead is connected to the  wrong block location. Correct any faulty connections before  installing the distributor block. Even though the idling fuel-air mixture is correct, there is a  tendency for oil to be drawn into the cylinder past the piston  rings, valve guides, and impeller shaft oil seal rings. At low  engine speeds, the oil combines with the soot in the cylinder  to form a solid that is capable of shorting out the spark plug. Spark plugs that are wet or covered with lubricating oil are  usually grounded out during the engine start. In some cases,  these plugs may clear up and operate properly after a short  period of engine operation. To check the induction vibrator, ensure that the manual  mixture control is in idle cutoff, the fuel shutoff valve and  booster pump for that engine are in the off position, and the  battery switch is on. Since the induction vibrator buzzes  whether the ignition switch is on or off, leave the switch off  during the check. If the engine is equipped with an inertia or  combination starter, make the check by closing the engage  mesh switch; if the engine is equipped with a direct-cranking  starter, see that the propeller is clear and close the start switch.  An assistant stationed close to the induction vibrator should  listen for an audible buzzing sound. If the unit buzzes when  the starter is engaged or cranked, the induction vibrator is  operating properly. Engine oil that has been in service for any length of time  holds in suspension minute carbon particles that are capable  of conducting an electric current. Thus, a spark plug will not  arc the gap between the electrodes when the plug is full of  oil. Instead, the high-voltage impulse flows through the oil  Spark plug operation can often be a major source of engine  malfunctions because of lead, oil, graphite, carbon fouling,  and spark plug gap erosion. Most of these failures, which  usually accompany normal spark plug operation, can be  minimized by good operational and maintenance practices.  A spark plug is considered fouled if it has stopped allowing  the spark to bridge the gap either completely of intermittently. Figure 4-48.  Carbon fouled spark plug.  Spark Plug Inspection and Maintenance Checking Ignition Induction Vibrator Systems Carbon Fouling of Spark Plugs Oil Fouling of Spark Plugs 4-30