[toolboxchev]

I am no quadrajet guru but here is my take. All your timing and ignition has to be gone through before you work on the carb. If I am correct most others here will confirm.

You have a high mileage motor, that being said a vacuum leek will cause some issues. Check your Vac Booster for brakes, these will be bad affecting idle, then check you vac advance. Either or can be bad with little noticeable or visual confirmation unless tested properly. These are diaphram operated and sensitive to ethanol. They can develop very small pinholes and again until tested properly for function can hold a mass of symptomatic problems.