1985 Chevy Blazer 4BBL Carb

Engine Performance problem
1985 Chevy Blazer V8 Four Wheel Drive Automatic

Hi;
I have a carb problem on a '85 full size blazer k-5 with the "H" motor 305 automatic tranny. The truck was purchased new and never abused or modified. Recently the 4BBL card stopped high idling upon start up, No matter how cold or how hot the motor is when you start the truck up it idles so low that the truck dies. It cannot be driven out of the driveway unless you keep pressing the accelerator. The rpm at start up is about 560's and when you shut down the truck it continues to run"diesels" What could be the culprit?

Thank you