One spark plug Burns black


Asked by Jan 06, 2017 at 12:19 AM about the 1967 Volvo P1800

Question type: Maintenance & Repair

B18 engine completely
restored running twin
Mikuni carburetors first
spark plug nearest the
radiator burns black all the
rest are chocolate brown

1 Answer


Overly rich fuel/air mixture, dirty air filter, too much driving at low speeds, or idling for a long time. Switch to “hotter” plug. (The higher the plug number, the hotter the plug.) Oil-fouled plug. Wet, black, oily deposits on insulator tip and electrodes.

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