OMC Stern Drives And Motors 1964-1986 Repair Manual., Page 37Get this manual
ENGINE SERVICE
Combustion The power delivered from the piston the crankshaft is the result of pressure increase in the gas mixture above the pistonThis pressure increase occurs as the mixture is heated, first by compression, and then (on the down stroke) by burningT burning fuel supplies heat raises tempe and, at the time, raises pressureActually, about 75 percent of the mixture in the cylinder is composed of nitrogen gas does not burn but expands when heated by the burning of the combustible elements, and it is this expanding nitrogen supplies most of the pressure on the pistonThe fuel and oxygen must burn smoothly within the combustion chamber take full advantage of this heating effectMaximum power would not be delivered the piston if an explosion took place, because the ent would spent in one sharp hammer-like blow, occurring the piston followInstead, burning place evenly as the flame moves across the combustion chamberBurning must be completed by the the piston is about half-way down maximum pressure will developed in the cylinder at the the piston applies greatest force the crankshaftThis will when the mechanical advantage of the connecting rod and crankshaft is at maximumAt the beginning of the power (as