been consumed, some of the excess oxygen unites with  some of the lead and some of the scavenger agent to form  oxygen compounds of lead or bromine or both. Some of  these undesirable lead compounds solidify and build up  in layers as they contact the relatively cool cylinder walls  and spark plugs. Although lead fouling may occur at any  power setting, experience indicates that the lead buildup  is generally confined to a specific combustion temperature  range. Combustion temperatures outside this specific range  minimize the lead fouling tendency.  from one electrode to the other without a spark as though  a wire conductor were placed between the two electrodes.  Combustion in the affected cylinder does not occur until, at  a higher rpm, increased airflow has carried away the excess  oil. Then, when intermittent firing starts, combustion assists  in emitting the remaining oil. In a few seconds, the engine is  running clean with white fumes of evaporating and burning  oil coming from the exhaust. Lead fouling of aviation spark plugs is a condition likely  to occur in any engine using leaded fuels. Lead is added  to aviation fuel to improve its anti-knock qualities. The  lead, however, has the undesirable effect of forming lead  oxide during combustion. This lead oxide forms as a solid  with varying degrees of hardness and consistency. Lead  deposits on combustion chamber surfaces are good electrical  conductors at high temperatures and cause misfiring. At low  temperatures, the same deposits may be good insulators. In  either case, lead formations on aircraft spark plugs prevent  their normal operation.  [Figure 4-49]  To minimize the  formation of lead deposits, ethylene dibromide is added to  the fuel as a scavenging agent that combines with the lead  during combustion. If lead fouling is detected before the spark plugs become  completely fouled, the lead can usually be eliminated  or reduced by either a sharp rise or a sharp decrease in  combustion temperature. This imposes a thermal shock on  cylinder parts, causing them to expand or contract. Since  there is a different rate of expansion between deposits and  metal parts on which they form, the deposits chip off or are  loosened and then scavenged from the combustion chamber  by the exhaust or are burned in the combustion process. Lead fouling may occur at any power setting, but perhaps the  power setting most conducive to lead fouling is cruising with  lean mixtures. At this power, the cylinder head temperature  is relatively low and there is more oxygen than needed to  consume all the fuel in the fuel-air mixture. Oxygen, when  hot, is very active and aggressive. When all the fuel has  Several methods of producing thermal shock to cylinder  parts are used. The method used depends on the accessory  equipment installed on the engine. A sharp rise in combustion  temperatures can be obtained on all engines by operating  them at full takeoff power for approximately 1 minute.  When using this method to eliminate fouling, the propeller  control must be placed in low pitch, or high rpm, and the  throttle advanced slowly to produce takeoff rpm and manifold  pressure. Slow movement of the throttle control provides  reasonable freedom from backfiring in the affected cylinders  during the application of power. Another method of producing thermal shock is the use of  excessively rich fuel-air mixtures. This method suddenly  cools the combustion chamber because the excess fuel does  not contribute to combustion; instead, it absorbs heat from  the combustion area. Some carburetor installations use two- position manual mixture controls that provide a lean mixture  setting for cruising economy and a richer mixture setting for  all powers above cruising. Neither manual mixture control  setting in this type of configuration is capable of producing  an excessively rich fuel-air mixture. Even when the engine  is operated in auto-rich at powers where an auto-lean setting  would be entirely satisfactory, the mixture is not rich enough. As a result of careless and excessive application of thread  lubricant, called antiseize compound, to the spark plug, the  lubricant flows over the electrodes and causes shorting.  Shorting occurs because graphite is a good electrical  conductor. The elimination of service difficulties caused  by graphite is up to the aircraft technician. Use care when  Graphite Fouling of Spark Plugs Figure 4-49.  Lead fouled spark plug.  4-31 Lead Fouling of Spark Plugs