Spark plugs carbon fouling

[smittykid]

I was just talking to a fellow that has a 29 Speedster for sale. He says after about 100 miles the 1st and fourth spark plugs foul. Two and three are OK. It's carbon fouling-not oil. He's running a MIller High Speed high compression head and a Stromberg 81 carb with an electronic ignition. He can't figure out why it is just the first and fourth plugs that foul. Any thoughts or is there anything else I should be asking? Thanks