Best blame so far has been ignition. Wouldn't have happened back when the car was younger and the gas did not evaporate. The best explanation I have gotten so far is that carb is empty, and when the fuel pump get first hint of gas into the carb it is vapor. The engine mis-starts and backfires up through one of the jets, around the step-up rods and lights at the needle and seat. It melts the solder and by that time I am trying to restart the engine and the fuel pump finally gets liquid in the bowl and it fills the float, because the hot float creates a suction as it cools, then it cools the solder and seals the float back up, because apparently liquid gasoline does not burn, just the vapor. Now the float is bad on one side and the engine only runs right on 4 cylinders because of the dual plane intake. Unless someone has a better Crystal Ball than the outside people I have quizzed about this.
Best workaround without modifying the car is going to be filling the vent tubes with fuel before starting when the car has been sitting for a while. Or dynamite.