[Eljaro]

Rick,
a dirty needle valve can cause that. The slightest rest of gasket, silicone or gas tank dirt can prevent the needle valve from closing.
But you should consider one more thing:
I used to have all four carbs pouring gasoline into the throats on idle and after shutdown. I could hear the gasoline boiling inside the bowls. So I installed a set of phenolic spacers, and made myself even a set of phenolic washers for under the screw heads and all was well again.
On top of that I took a a return line from the second output on my Holler fuel pressure regulator and fed it back to the fuel line before the mechanical fuel pump, with a 115 air corrector jet pressed inside the tee fitting to limit the flow of the feedback.
Now the pressure stays up and is more stable but as soon as I shut off the engine the fuel pressure is relieved through the bypass in a few seconds. No more residual pressure on the needle valves, no more gasoline pouring into the throats.