Thee conditions needed for an engine to run correctly consist of fuel, electricity and compression. Compression has to exist in the outboard motor's cylinders in order to compress a combination of fuel and air into an explosive mixture. A hot spark drives the piston down, which in turn drives the main crankshaft for power. The piston must then rise to expel exhaust gases, compress and complete another power stroke. The piston, rings and valves must form a perfectly tight seal while doing this, or compression drops off and poor performance results.
Secure the boat to the pier or dock if you're performing this test in the water. Place the manual motor tilt or electric trim in the full down, vertical position. If you have a key lanyard in your ignition, pull it. If not, disconnect your main ignition wire, then use a socket to remove the negative battery cable from the battery. Shut off the fuel supply valve.
Unclasp the snaps to the to top engine cowl case, or use a socket to remove the bolts, depending upon the fasteners. Set the cowl aside. If you have a multiple cylinder engine, unplug all of the spark plug wires from the spark plug tips. Use pliers to bend sections of coat hanger wire to fit into each wire connector and ground the other ends of the coat hanger sections to the engine block. Use a socket and wrench to remove all spark plugs from the engine.
Screw the adapter hose of a compression gauge into a spark plug hole. Tighten the adapter with an end wrench, but not overly tight. If your motor has an electric starter, temporarily connect the negative battery cable by hand, reconnect the ignition wire connector and insert the lanyard key. Make sure the choke is off.
Have an assistant turn the key to the "Start" position so the engine turns over, while holding the throttle in the wide open position. Let the engine turn over five to seven times then stop. Read the psi on the gauge and record it with pen and paper. Remove the compression gauge, screw it into the next cylinder and repeat same procedure. For a pull-rope recoil starter, you must pull the rope four or five times for each cylinder and record the psi.
Check your owner's manual under the section for compression specification requirements for your outboard motor. The psi listing in the manual will be the minimum allowable number for your engine. If any of the engine cylinders fall lower than 15 psi, you have compression deficiency in that cylinder, resulting from worn rings, a bad valve seat or worn valve faces.
Remove the coat hanger wires from the plug wire connectors. Screw the spark plugs back into their respective cylinders and initially tighten them with the socket and wrench. Refer to your owner's manual for the proper torque number on the plugs, and tighten them with a torque wrench.
Push the plug wires back onto the plug wire tips, seating the rubber boots firmly. Tighten the negative battery cable with a socket. Replace the engine cowl case and snap the clasps. Turn the fuel selector valve back on and replace the lanyard key.
