1978 Johnson 175, 200, 235 HP Outboard Service Manual, Page 68Get this manual
DESCRIPTION The power head consists of the cylinders, pistons, connecting rods, crankshaft, and crankcaseThe power head has two banks of horizontally-moW1ted cylinders in "V" formation 90 apartEach cylinder delivers one power impulse per crankshaft revolUtion, thus giving one power impulse at every 60 degrees of crankshaft rotation
Thes ports connect with the crankcase where fuel and ir 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 th 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 StrokeAt th 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 5000 or more times every minuteSee Figure 5-1CPower Stroke