1976 Evinrude 40HP outboards Service Manual, Page 33Get this manual
SPARK PLUGS Spark plugs having the proper heat range are very important for peak operation of the motorSpark 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 plugAttach leads to spark gap tester with gap set to 716"See Figures 4-10 and 4-10A(Remove spark plugs for easier cranking Crank engine with key switch or ropeSpark strong and steady firing each cylinder separately, ignition system is goodCheck spark plugsNo spark, check system with Stevens S-80 or MercO-Tronic M-80 neon test light
Figure 4-9HOT and COLD Plugs Selection of the correct spark plug depends on the type of service to which it is subjectedUnless the spark plug is properly suited to the motor, trouble may arise which might be interpreted 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 control with one plugSpark plugs furnished with the motor are selected for average serviceSee Section 2, Specifications