I have a strange idling issue I could use some help with.

This is on my 68 400 auto car.
The car ran like a top until last week. Took her out and when approaching a red light as I let my foot of the gas I could tell it will die on me. It didn't sound right all of a sudden.

The RPM at idle changed from about 700 to 500 or less. I kept my foot on the pedal to keep her running. I continued my drive for another 5 miles until I decided to head back home. It ran great once I was cruising but idle was rough and low.

I did most of the obvious so far, inspected the dizzy cap, checked timing, dwell, changed the points, plugs, checked the ignition coil. The carb was rebuilt a year ago so it should be fine still but haven't looked into it yet. All the plugs were tan, so good there.

Any help would be appreciated.

Thanks in advance.

Do you still have the idle stop solenoid on your engine? It's a quick check to see if it's working.

It is designed to eliminate run-on/dieseling when the car is shut off. When running it keeps the idle up and when shut down it drops idle down.

Yes I do and it is working. The car is not dieseling when shutting off, it is just running kinda rough and low RPM at idle, still runs normal driving.

Sometimes the springs on the idle mixture screws get weak, or just vibrate out. Try adjusting the mixture screws. Doesn't sound like a spark or timing issue.