Percolation? Heat soak? Don't drive during summer??
What did the plugs look like?...
Always depended on the tune up. But final tune never showed awful plugs. Wish I had pics. But they were never bright white nor super dark. It was also decent cylinder to cylinder.
I could list all the changes, from the larger two core alum radiator to the clutch fan (which replaced the direct drive fan), to the heater core bypass and even trying different thermostat temps (including none). When the weather was hot, it just didn't operate correctly.
The only reason I even brought up my issues was because I didn't want to see a lot of effort get spent in vain, because somewhere someone could probably have made my combo work, but the typical suggestions did squat for me. I really think cooler air to the carb would have done a lot. Looking back, I wished I had tried it.