Random Carburetor dripping seeping drooling

Agreed, and I do have a cockpit fuel shutoff valve located in the fuel supply line to the mechanical engine-driven pump, but have not closed it when parking in my hangar in the past. Wouldn’t excess bowl fuel be forced out through the ambient air vent tube rather than up and into the carb throat?

Most likely the the float valve isn’t working 100% and flooding overnight with the constant pressure of the fuel from the high wing tank. Using the shutoff valve after engine shutoff will solve this issue.

"Wouldn’t excess bowl fuel be forced out through the ambient air vent tube rather than up and into the carb throat?"

If your carburettor (s) are flooding, when the aircraft sits for an extended period, the route that the excess fuel will take will be governed by the angle of the engine and the way the float chamber breather tube (s) have been installed.

#  In aircraft with a tailwheel the engine angle is significantly diffrent to a nose wheel. 

#  Breather tube installation can vary from short downward angled pipes to longer upward curved pipes and variations between.

So riddle me this folks- if leaving one’s fuel shutoff valve open on a high wing aircraft when parked overnight can possibly override a carb float bowl fill valve-which is designed to function under 5.8 psi continuous pressure-and create a fuel leak- how could this occur when my fuel pressure gauge indicates 0.7 psi static fuel pressure to the carbs under said circumstances? 
I could agree with a small particle of debris lodged between the float bowl valve tip and seat causing such a leak, but not 0.7 psi of constant pressure. 
On a side note, when I was inspecting things the day I found the leak, when I removed the air filter I found fuel seeping from both the idle air valve and the needle jet air duct at the air filter opening. Looking at the Bing diagram in the MMH, I don’t see any common intersection of these air passages. Hmmm. Anyway, as of this morning, all is dry and well.

JIm

The "intersection" is the float bowl.  Both the idle and needle jet are feed from it.  I think you just have to accept that your needle, or both needles, are not fully closing off sometimes.  Shutting off the main fuel supply is great but a full float bowl has only to get a tiny bit more to start to drip as you state.  At least with the fuel shutoff they should not flood the hanger floor.  If it is a major issue for you then perhaps it i time to replace them. 

Remember gravity sucks. When running the needle is going from on to off constantly.  When  you stop if it is clean and not worn, including the needle seat, then no fuel will leak past.  The check for leakage is found in the maintenance manual heavy.

Cheers