14 BASIC POWERBOATING. SAFETY & RESCUE FOR SAILING INSTRUCTORS 3. Outboard Motors


KEY CONCEPTS E Two- & four-Strokes


E Maintenance


E Outboard Motor Parts E Fuel/Oil Discharge E Starting Procedure


TWO-STROKE & FOUR-STROKE


Outboard motors can range in size from small two-horsepower (hp) units to massive 300 horsepower (hp) engines. They can be either two-stroke or four-stroke (four-cycle). Two-stroke outboards use oil mixed into the gasoline to lubricate the engine. Each “compression” stroke of a piston is followed by a “power” stroke. As the power stroke comes to its end, a new mixture of gasoline/oil and air enters the cylinder, and exhaust gases are forced out along with some of the new incoming mixture. These outboards have been regarded as serious polluters, but recent innovations, such as oil injection and the replacement of carburetors with fuel injection, have reduced pollution to a level almost comparable to four-stroke models.


Four-stroke outboards are lubricated by oil in the crankcase, similar to an automobile engine, and produce less pollution than two-stroke outboards. An “intake” stroke, which brings in a mixture of gasoline and air, precedes each “compression” stroke, which is followed by a “power” stroke. The “exhaust” stroke, which forces out the exhaust gases, completes the cycle, and the next cycle starts again with the “intake” stroke.


As concerns about air and water pollution have increased, manufacturers have developed cleaner-running outboards. Some states have stringent pollution requirements for reservoirs and inland waters that may affect the use of your boat.


PARTS OF AN OUTBOARD MOTOR


Primer pumps are replacing the traditional choke in modern outboards. On electrically started engines, pushing in the key activates a “primer” pump that injects a small amount of fuel into the cylinder. More recently, manually activated primer pumps have been appearing on smaller outboards with manual pull-cord starting. At first glance, these can be confused with a traditional choke knob, but they are usually accompanied by a decal on the face of the engine listing instructions for use. Manual primer pump knobs must be pulled out and then pushed back in to inject the fuel. Some retract automatically, while others must be pushed. If left out, the engine will not start.


Table of Contents


Air/fuel/oil mixture


Crankshaft


Compression stroke


Power stroke


ABOVE: A two-stroke engine fires once every revolution of the crankshaft.


BELOW: A four-stroke engine fires once every second revolution of the crankshaft.


Air/fuel mixture


Crankshaft


Compression stroke


Intake stroke


Spark plug ignites


compressed mixture.


Power stroke


Exhaust stroke


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