If the coolant temp is too low , the mixture too rich or the engine worn and burning a high ash oil. It will foul any plug. Putzing around at light load and with coolant temp under 195F is a common problem.
All too often the mixture is too rich from 35 years of dirt in the air inlet pipe work or aircleaner. A choke blade that is partly closed even through the choke knob is fully inserted is all too common.
If it's oil fouling. Use low ash SF-2 Shell Rotella T1 50 weight intended for two stroke Detroit Diesels.
Use solid core wires and Autolite 3116 plugs.