Engine dies when weather is wet

[Husker]

My 68 Chevy
250 6cyl
single barrel carb
HEI ign (new wires, new cap and rotor)
Stock air cleaner and filter is good, has heat tube down to manifold
Manual choke
manual trans
On my way to work about 7 miles or so about 2/3rds way there the last couple of days it has been raining or heavy fog, when I come to stop or just in neutral coasting the engine is sputtering, idling low and it will die. Starts back up OK but if I want to continue running I have to give it gas or pull choke out a bit. I cleaned the Distributor cap with electrical parts cleaner and used carb cleaner around carb and manifold. No apparent vacunm leaks. Not sure what is causing this, any help would be great.
Thanks
Ron