[Brentco]

When you start the car to run it for five minutes, what’s your procedure to get it running without a choke? Are you pumping the pedal a couple times? And how often do you take it out for actual drives/hard drives? It may be as simple as the engine is not being run long enough or hard enough to burn off the initial fuel from starting it. Pump shots deliver a lot of fuel and the engine takes a long time to burn it off, probably longer than you’d think. A wideband would help you see how rich it’s running during this initial period.

Given that the plug-fouling issue has been happening for a long time and has been the same through different iterations of the engine and carb/timing changes, I’d suspect the one constant — the five-minute idle sessions. Maybe let it idle longer and/or rev it higher when you need to back the car out of the garage.