Outboard spark plugs should match the exact type, heat range and gap listed in your owner’s manual or on the engine decal, and be checked or replaced at the interval the manufacturer gives, often yearly or after a set number of hours. Reading a used plug tells you a lot about the engine: light tan or gray is normal, black and sooty means too rich or too much idling, wet and oily points to flooding or too much oil, and white or blistered means the engine ran lean or hot.
Choosing the right plug
Use the plug the engine maker specifies. Many outboards call for a particular resistor or marine-rated plug, and a plug with the wrong heat range can foul or overheat. Cross-reference charts help when your manual lists only one brand, but confirm the heat range matches before you buy an equivalent.
Reading outboard spark plugs
| Plug looks | What it usually means |
|---|---|
| Light tan or gray, dry | Normal combustion |
| Black, dry and sooty | Rich mixture, choke left on, long idling or trolling, weak spark |
| Wet with fuel or oil | Flooding; on two-strokes, too much oil or a cold, idling engine |
| White, glazed or blistered | Lean or overheating; stop and diagnose before running hard |
| One plug differs from the rest | A problem on that cylinder: injector, carburetor circuit, compression or coil |
A white plug is the one to take seriously; lean running can damage pistons. See outboard overheating and outboard loses power at full throttle.
Why two-strokes foul plugs
Two-stroke outboards burn oil with the fuel, and long periods of idling or trolling let deposits build up on the plugs. Running the engine at higher rpm for a while helps clear light fouling. See 4-stroke vs 2-stroke outboards.
Changing them
- Let the engine cool. Blow debris away from the plug wells first so nothing falls into the cylinder.
- Check the gap against the manual; do not assume a new plug comes gapped correctly.
- Thread plugs in by hand, then tighten to the torque in the manual. Follow the plug maker’s advice on anti-seize, since many plated plugs are meant to go in dry.
- Check boots and leads for cracks and corrosion, especially in salt water; a little dielectric grease inside the boot helps keep moisture out.
Testing for spark
An inline spark tester shows whether each lead is firing without pulling plugs while cranking. Never crank with a lead hanging loose near fuel vapor. If there is no spark on any cylinder, check the kill switch and lanyard first; see outboard kill switch.
Related
For no-start and rough running, see outboard won’t start, outboard runs rough and outboard won’t idle. For the wider service schedule, see outboard motor maintenance and how to winterize an outboard. Car owners can compare bad spark plug symptoms.
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