1971 Evinrude StarFlite 100 HP Outboards Service Manual, PN 4753, Page 45Get this manual
DESCRIPTION
The power head consists of the cylinders, pistons, connecting rods, crankshaft, and crankcaseThe power head has two banks of horizontally-mounted cylinders in "V" formation 90 apartEach cylinder delivers one power impulse per crankshaft revolution, thus giving one power impulse at every 90 degrees of crankshaft rotation
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 exhaust-intake-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 The inrushing charge of the fuel-air mixture helps in ejecting the last of the exhaust gases from the cylinderSee Figure 5-1A, 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 5-1B, Compression Stroke