1978 Johnson 55 HP Outboards Service Manual, Page 58Get this manual
OEseR IPTION The power head consists of the cylinders, pistons, rods, crankshaft, and crankcaseThe power head has two horizontally mounted cylindersAlternate firing order is used so that each cylinder delivers one power impulse per crankshaft revolutionSee pages 5-2 and 5-3 for two stroke theory
and Exhaust