Basic Outboard Maintenance: Changing Spark Plugs

Changing the spark plugs on an outboard engine is a simple “do-it-yourself” maintenance task that can be done with even the most basic mechanic skills.

Your outboard’s owner’s manual will tell you how often the spark plugs should be changed, but it’s recommended that the plugs be switched out at least once a year. Most boat owners do so at the beginning of the boating season. Here's how to change the spark plugs on an outboard motor.

Tools and Parts – Outboard Spark Plug Replacement

  • Ratchet
  • Ratchet extension bars
  • Spark plug socket
  • Spark plugs

Buy outboard spark plugs

Buy outboard spark plug wires

NOTE: The location of the spark plugs varies depending on the outboard, but in general the layouts and the steps to change them are the same or similar to one another. 

Step 1. Remove the engine’s cowling, which is usually secured by latches at the front and back of the engine.

Outboard engine spark plugs replacement

Step 2. Note the order of the plug cables and remove them from the spark plugs.

Outboard motor spark plugs change

PRO TIP: Some plug cables are numbered, but if they’re not, number small pieces of masking tape and tag the cables accordingly to help you remember their order.

Step 3. Remove the spark plugs using the ratchet, extension bar and plug socket. Use a swivel head universal joint with the ratchet if the lowest plug is hard to reach.

Outboard motor spark plug replacement

PRO TIP: Inspect the old plugs before installing the new ones. The plugs can tell you a lot about the health of your outboard. If any of the plugs are oiled or overheated, have the motor inspected immediately. An even coating of black soot is normal, but nothing more. A plug covered in oil is an indication of a failing gasket or piston, and one covered in white cinder debris indicates overheating caused by incorrect engine timing or a mixture of fuel and air.

Watch the video above to see how to test outboard spark plugs.

Step 4. Replace the old spark plugs with new ones, but don’t overtighten them. Follow the directions on the package for how much to tighten the plugs.

Outboard engine new spark plug

PRO TIP: Generally plugs should be finger tightened, then given an additional half-turn to full-turn with the ratchet.

Outboard engine spark plugs change

Step 5. Replace the plug cables in the order in which they were removed.

Spark plug replacement outboard motor

PRO TIP: If the cable doesn’t clip back onto the plug or seems loose, crimp the metal clip inside the plug cable boot with a pair of pliers.

Step 6. Replace the outboard’s cowling, then fire up the engine to check that it’s running correctly. If the engine misfires or runs rough, the plug cables were reattached in the wrong order. Remove the cowling and reattach them in the correct sequence.

PRO TIP: Always have water running into the outboard’s intake, and use an engine flusher when running the motor on land. 




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