69 RR 383 with 4615S Carter AVS. I have had two brass floats fill with fuel a couple years apart. Both times the top gasket shows signs of fire and one float will have dark marks all over it. The carb was restored by a respected person for concours use when the car was completed 5 years ago. The car only uses non ethanol fuel from a fuel distributor. If it sits for more than a couple days the bowls are empty and it has to crank for 15-30 seconds before firing. Less than that, it fires immediately but is cold-natured.

My take is that after sitting when I try to start the engine it "pops" back through the carb and starts a flash fire in one of the bowls. This melts the solder and then the float fills and then the fire is out. I then get clouds of black smoke and running on 4 cylinders.
Is this possible?
What allows it to do this?
How do I stop it?