""have tried several different carburetors (not because of this problem, just because I have other 318s and carburetors get swapped around during various adventures) and the problem is always unchanged. So things like air/fuel mixture aren't real logical unless every 318 carb I have has the same problem, also unlikely cuz they run the gamut from 60's Stromberg to 70's Carter to 80's Holley models.""

Back when the 318 carbs were designed and tuned from the factory, I would guess that there was a different type of fuel than what we use today. If you tried several different 318 carbs, I would assume that they would all give the same results.

Todays fuel (E10) needs a richer A/F mixture than when these 318 carbs were designed. If the engine starts to warm up at 55-60mph, then gets even warmer above that speed, then I would at least try to richen up the jets to see if that helps out the temp at those speeds.

I have had to richen up my A/F mixture just over the last couple of years. I am assuming that this is due to change in the fuel at the pump over the last couple of years. I can't imagine trying to run a stock 318 carb on todays fuel without changing the carb setup.