BARIUM/CALCIUM FOULING. The use of high-ash lubricating oils affect the performance of an engines spark plugs. If you suspect this operating problem, check the engine manufacturer's lube oil specifications. If ash content of lube oil used does not conform to manufacturer's specifications, change to a recommended lube oil. If engine provides power cylinder lubrication, check power cylinder lubrication rate to assure that lubrication rate is within specifications. If the lube oil is one recommended by engine manufacturer's specifications, check that cylinder liners are not scuffed or scored. Next, check engine lube oil consumption history and then check the piston rings. If lube oil ash deposits indicate an excessive accumulation in one quadrant of firing end, lube oil leakage down intake valve guides may be indicated.

CARBON DEPOSITS.
Fluffy black carbon deposits usually indicate too rich a fuel/oxygen mixture. If a change in the mixture to a leaner setting fails to remedy this carbon fouling, try using Stitt's next hotter heat range spark plug.