Why plugs are worth reading every time

Spark plugs are free diagnostic data. Any time plugs come out — tune-up, compression test, winterization — lay them out in cylinder order and read them before tossing them. On a multi-cylinder engine, differences between cylinders matter more than any single plug's appearance.

The reference conditions

• Light tan/gray deposits, dry: healthy combustion. This is baseline.
• Black, dry, sooty: rich mixture or cold running — carb adjustment, choke/enrichener stuck, thermostat removed or stuck open, or long idle-only use.
• Black, wet, oily (four-stroke): oil in the cylinder — rings or valve guides; do a compression/leakdown before quoting.
• Black, wet, oily (two-stroke): can be normal-ish with over-oiling or trolling use — but compare cylinders; one oily plug among clean ones points at that cylinder (rings, or an injection distribution issue).
• Chalky white, blistered insulator, or specks of aluminum: lean and hot — stop and find out why before the piston pays for it. Think air leaks (carb base gaskets, crank seals on two-strokes), water in the fuel leaning a cylinder, or blocked jets.
• Rusty, wet with water, or steam-cleaned spotless on one cylinder: water intrusion — head gasket or exhaust (see the manifold/riser article for sterndrives). A single suspiciously clean plug is a water alarm, not a compliment.
• Bridged gap or melted electrode: debris or detonation — investigate fuel octane, timing, carbon buildup.

Replacement practice

1. Replace with exactly the plug type the service manual lists — heat range matters in marine use; "close enough" auto-store substitutions cause fouling or overheating.
2. Check the gap against the manual before installing, even on pre-gapped plugs.
3. Thread by hand first — aluminum heads strip easily; torque per the service manual, and use anti-seize only where the manual says to.
4. Inspect plug wires/coil boots while you're there; carbon-tracked boots re-foul new plugs.

Common mistakes

• Reading plugs after idling to the dock (idle fouling masks the cruise story — the ideal read is after a run at load)
• Mixing up cylinder order and mis-assigning the finding
• Replacing fouled plugs without fixing the cause — the new set fouls on schedule

When to walk away

Aluminum speckle on any plug means the piston is already shedding metal — stop selling tune-up parts and quote a proper teardown assessment.

Safety: remove all plugs only with ignition disabled; on a hot engine let it cool — plugs in aluminum heads gall when removed hot.