Hi Swift Fox, and thank you for your insight. It is not lead that fouls spark plugs. If this were the case it would not be deemed suitable for aviation use. Improper fuel/air mixtures and weak sparks are the main culprit with fouled plugs. A little clean burning 100 LL in my opinion, when used in a tiny hit and miss engine, is likely considerably better to breath than than poison we burn in our automobiles. Still.... all gas engines big and small should be run in a properly vented area or outdoors. Aircraft fuel blends also have greater evaporative qualities in cool environments than automotive fuels.
Gil
Combustion chamber temperature also plays a role too, as that was the reason i mentioned it as i assumed these engines had quite a low combustion temperature which could possibly have an effect but on thinking it over i guess given the short times they actually run it wouldn't have much of an effect at all. I'm quite familiar with 100LL since it what fuels the aircraft i fly. I agree modern automotive fuels that contain ethanol and other additives are quite bad.
Glad to hear you run yours in a well ventilated space or outdoors.