1979 Evinrude 4 HP Outboards Service Manual, PN 5424, Page 51Get this manual
DESCRIPTION The powerhead consists of the cylinders, pistons, rods, crankshaft, and crankcaseThe powerhead has two cylinders horizontally mounted in vertical planeThe firing order is combined so that each cylinder delivers one power impulse per crankshaft revolution, thus giving one power impulse at every 180 degrees of crankshaft rotation See Figure 5-1