Spark Plug Reading Guide
"Reading" the plugs reveals mixture, heat range, and engine condition. Run the engine under load, cut the ignition, and coast to a stop before pulling the plugs. Compare all cylinders.
| Appearance | Condition | Likely Cause |
|---|---|---|
| Light tan / gray, slight wear | Normal | Correct heat range and mixture |
| Black, dry, sooty | Rich / carbon-fouled | Rich mixture, sticking choke, short trips |
| Black, wet, oily | Oil-fouled | Worn rings/guides or too-cold plug |
| White/blistered, eroded electrode | Overheating | Too-hot plug, lean, over-advanced timing |