1977 Evinrude 4HP Outboards Service Manual, PN 5303, Page 45Get this manual
DESCRIPTION The power head consists of 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 of crankshaft rotationSee Figure 5-1
E XHAUST PORT
INTAKE PORT FUELAIR MIXTURE
E XHAUST GASES
THEORY OF OPERATION
Two-cycle engines used on outboard motors require only two piston strokesone up, one down, to effect crankshaft revolution and to complete the exhaustintake-compression-ignition sequence that produces powerIn two-cycle engine, ignition of the fuel-air mixture occurs as the piston reaches the top of each strokeThe explosion drives the piston downwardToward the end of the downward stroke, ports which lead to the exhaust system are uncoveredThe exhaust 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 been induced by 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, A, Fuel Intake and Exhaust