DIY Johnson Outboard Compression Test
- 1). Hook a water hose to the flushing connection, if the motor has one, or to a flushing attachment, if it doesn't. Put the flushing attachment over the inlets on the exhaust tunnel for the motor's cooling water. Turn the hose all the way on. Start the motor.
- 2). Allow the motor to reach its normal operating temperature. Turn the motor off. Take the "Emergency Stop" lanyard clip off the throttle to prevent the motor from starting. Take the spark plugs out of the motor with a plug wrench, keeping them in order.
- 3). Screw the compression gauge into the No. 1 spark plug hole. Set the throttle to the "wide open throttle" position. Crank the engine with the ignition switch for five seconds, or pull the starting rope five times -- the motor won't start because the spark plugs have been removed. Record the compression, in pounds per square inch, that the compression gauge registers.
- 4). Reset the gauge to zero. Repeat this process five times, recording the result each time. Pick the highest compression recorded for the cylinder and strike the rest out.
- 5). Calculate the difference between each cylinder's compression from your notes. The compression in any one cylinder should not vary from the compression in the other cylinders by more than 15 percent, which is the limit established by Johnson.
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