1974 Johnson 40 HP Outboard Motors Service manual, Page 32Get this manual
SPARK PLUGS Spark plugs having the proper heat range are very important for peak operation of the motorThis motor is designed to operate with Champion UL4J spark plugsSpark plugs are classified according to the temperatures at which they are designed to operate, HOT or COLDSee Figure 4- 9 Remove spark plug leads from spark plugs by grasping rubber boot and twisting counterclockwise while pulling lead off leads to spark gap tester with (Remove spark plugs for easier engine with key switch or rope startSpark strong and teady firing each cylinder separately, ignition system is goodCheck spark plugsNo spark, check system with Stevens S-80 or Merc O- Tronic M-80 neon test light )
Figure9HOT and COLD Plugs Selection of the correct spark plug depends on the type of seriVice to which it is subjectedUnless the spark plug is properly suited to the motor, trouble may arise which might be interpre ted as carburetor difficultyVery low trolling speeds will tend to foul plugs due to the oil not burning from the coreHowever, at full throttle with hotter plug, the operating temperature may be too high, resulting in preignitionAn extreme temperature range will be difficult to