1971 Johnson 4HP Outboard Motors Service Manual, Page 36Get this manual
DESCRIPTION The power head consists the cylinders, pistons, rods, crankshaft, and crankcaseThe power head 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 crankshaft rotationSee Figure 5-1
Fuel Intake and Exhaust
EXHAUST PORT
INTAK POR FU LAIR MI TUR E
EXHAUST GASES
THEORV OF OPERATION
Two-cycle engines used outboard motors require only two piston strokesone down, to effect crankshaft revolution and to complete the exhaustintake-compression-ignition sequence that produces powertwo-cycle engine, ignition of the fuel-air mixture occurs as the piston reaches the top each strokeThe explosion drives the piston downward Toward the end of the downward stroke, ports which lead to the exhaust system are uncoveredThe haust gases flow into these ports, thus reducing the pressure in the cylinderAt almost the same time, intake ports are openedThese ports connect with the crankcase where fuel and air mixture has induced carburetionThe downward motion of the piston compresses this mixture in the crankcase and forces it through the intake ports into the cylinderThe inrushing charge of the fuel-air mixture helps in ejecting the last of the exhaust gases from the cylinderSee Figure 5-1, Fuel Intake and Exhaust