How to Test Compression on a Small Outboard
A compression test on a small outboard engine is an important diagnostic method to give you some direction when you're working on your motor.
Here's how to do a compression test on a small outboard engine.
Tools Needed - Small Outboard Compression Test
- 5mm socket
- 21mm wrench or deep-welled spark plug socket
- Compression test set
NOTE: We did our compression test on a Johnson 6HP outboard motor, but the same or similar steps may be applied to your small outboard.
Check out our Johnson 1978 6R78B outboard cylinder and crankcase parts diagram for additional reference.
Johnson 6HP Outboard Compression Test
Step 1. Rotate the latch on the bottom of the powerhead at the back, from horizontal to vertical, to unlock the cowling and allow you to lift it off.
Step 2. Remove the boots from the two spark plugs by pulling them off.
NOTE: The boots are already removed from the plugs in the picture below.
Step 3. Remove the first (top) spark plug. Use a 21mm wrench or a deep-welled spark plug socket and ratchet for this.
Step 4. Choose the adapter from your compression test kit that fits your spark plug opening and screw it into place. Follow the instructions that come with your kit to attach the compression tester to the adapter.
Step 5. Set the throttle wide open and then pull the pull cord six or seven times to test the compression. Next, check the gauge on the compression tester.
NOTE: Johnson's specs are for the engine's compression to be between 85 and 90 PSI, so this cylinder passes.
Step 6. Remove the adapter from the spark plug opening and replace the spark plug.
PRO TIP: Now is a great time to change out the spark plug if necessary.
Step 7. Remove the spark plug from the lower cylinder and repeat the process, pulling the starting cord the same number of times as the top cylinder.
NOTE: In our case, we again found our cylinder at about 85 PSI, within Johnson's tolerance.
Step 8. Remove the compression tester and replace the spark plug.
Looking to perform a compression test on a larger outboard engine? Watch the video below to see how to do a compression test on a Yamaha F225 motor.