1968 Evinrude Ski-Twin, Ski-Twin Electric 33 HP Outboards Service Manual, Page 39Get this manual
DESCRIPTION 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 crankshaft revolutionSee Figures 5-1 and 5-2
E XHAUST PORT EXHAUST GASES
lNTAKE PORT
OF OPERATION
Two-cycle engines used outboard motors require only two piston strokesone 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 uncoveredThe haust gases flow into these ports, thus reducing the pressure in the cylinderAt almost the same time, intake ports openedThese ports connect with the crankcase where fuel and air mixture has been 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 Fuel Intake and ExhaustAs the piston begins its upstroke, it closes the intake and exhaust ports and begins to compress the fuel and air mixture trapped in the cylinderThe upward travel of the piston also reduces the pressure in the crankcase compartmentThe resulting suction opens leaf valves which admit additional air and fuel from the carburetor into the crankcase, thus preparing the next cylinder chargeSee Figure Compression Stroke