1968 Evinrude Starflite 100 HP outboards Service Manual, Page 43Get 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 apartThe firing order is combined so that each cylinder delivers one power impulse per crankshaft revolution, thus giving one power impulse at every 90 degrees of crankshaft rotation
As 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 StrokeAt the top of the piston stroke, the compressed fuelair mixture is ignited by timed spark and the cycle begins anewIn an outboard motor engine running at full throttle, this cycle may be repeated 4000 or more times every minuteSee Figure 5-1C, Power StrokeCRANKCASE AND CYLINDERS To provide equal fuel vapor distribution to the four cylinders, the crankcase is divided into four equal areas, and each is sealed off from the others through the use of compression type sealing rings on the crankshaft websAll four crankcase distribution chambers must be isolated from each other at all times to prevent compression leakage between cylindersSee Figure 5-2CYLINDER HEADS There are separate cylinder heads for each bank of cylindersThe cylinder head covers are numbered to correspond to the ignition firing orderThe starboard cylinder head cover is numbered upper and