1969 Evinrude 40 HP Big Twin, Big Twin Electric Lark Service Manual 4596, Page 34Get this manual
A21X spark plugsSpark plugs are classified according to the temperatures at which they are designed to operate, HOT or COLDSee Figure 4-2 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 core However, 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 motors are:
1Champion: J4J 2AC: M42K 3Auto-Lite: A21X
Figure 4-4Pulling Flywheel eRemove armature link, spring and washerLoosen four Phillips head screws attaching magneto armature plate to power headSee Figure 4-6Lift magneto assembly from power head