What fuel should I run in my 340?

Is the intake exhaust heat blocked off or running an Air gap intake? In cool weather the plugs can foul out. Fuel needs heat to burn, carbs are different than FI. Racing or hot summer weather heat isn't needed, in the winter on the street its a big help.

Plug wires wear out also, they need to be replaced after so long. If you have a spare carb you could swap carbs, this gas can loosen up crud and plug up the idle passages

Once running right you should not really feel the octane at idle.