1971 Evinrude 40HP outboards Service Manual, Page 31Get this manual
bTwist leads off spark plugs counterclockwiseDisconnect lead from vacumn cut-out switchc Remove flywheel nut and three flywheel screwsUsing Flywheel Puller (Special Tool #378103), pull flywheel from crankshaftSee Figure 4-4 1524
Figure 4-2HOT and COLD Plugs spark plugsSpark plugs are classified according to the temperatures at which they are designed to operate, HOT or COLDSee Figure 4-2Selection 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 pre-ignitionAn extreme temperature range will be difficult to control with one plugSpark plugs furnished with the motor are selected for average serviceSpark plugs recommended for the 40 HP model are: 1Champion: J4J 2AC: M42K