1983 Chevy 350 engine idle issue

[harleyphil8]

I have a 1983 Chevy C 10 with the 350 in it. It ran fine up until yesterday, when at the parts store it started to die at idle. Got it home, and today found that the truck will not run at idle. It will start when throttle is increased, however it runs rough, and the exhaust emits gray/black smoke. As soon as throttle is released, the engine immediately dies. I changed the spark plugs, checked the fuel filter, and i am stumped. The carb is still squirting fuel. The number 1 and 3 cylinder spark plugs had some deposits on them, but not much. I need some help here, thanks for your time.