[JAM1775]

Since you are running steady and strong you probably do not have a vacuum leak but a slight one should "lean" you out mostly instead of fouling you to the fat side. I still agree to check the intake gasket. What if those are the ports tightly sealed while 1/5 have a slight leak? Fix the leak and all four fronts could become fat. Then drop in primary jets.
How are those contacts in your distributor cap ? Are all 8 contacts equally carboned ? Could there be arching in the cap or in the wires ? Pull your carb and check your intake runners. Some dual planes criss cross like my Weaind stealth 1/6, 2/5, 4/7, 3/8 etc. Others may be symmetrical like 2/6. 2/6 may actually be fed by the same jet ! That would be easy if your carb was a bit screwy .Drop one jet size in that one corner to feed 2/6 a bit leaner. With My Weaind both front jets dropped would lean out 1/6/2/5. If 1/5 had a slight leak that could cause my 2/6 to foul down the road because my rich primary jets are compensating for the vacumm leaks in the front ports of the intake . Yes...it happened to me. It took a while to figure out though!