The throttle return spring attaches to a bracket off the left rear intake bolt as seen below. The spring in the pictures is a stronger than factory spring I had run on mine for some time. I believe I am not running the factory spring that is a bit weaker. I often see this bracket missing on parts cars that have went through intake replacement and someone forgot to replace the bracket or lost it and have attached the spring to something else. Check to see if you have your return spring setup in this manner. If you need that bracket, I have a ton of them... for $5 + s/h. I also have an assortment of springs.
Thanks again Jason, I do have the bracket on the intake but the one on the carb is different.
That's an Edelbrock carb you see there so ignore it. On both the Edelbrock, and stock Carter AFB and Rochester 4GC carbs, the throttle rod goes through the throttle plate on side of carb, and the spring simply goes through a hole through the rod. That is there is no additional bracket.
I added those last two pictures to our Carb Help Article at http://6364cadillac.ning.com/profiles/blogs/carburtors-used-in-1963...
Also on AC equipped cars check that the idle speed-up control on right side of carb is working properly. When the AC is on, it shuts off vacuum to the actuator, allowing the plunger to extend out and push the throttle lever causing idle to increase to 900 to 950 rpm (if set to factory specs). When the AC is off, you should have full vacuum to the actuator pulling it back in. If you simply disconnect that vacuum line it will allow the plunger to push the idle up as if the AC were on. I remember way, way back on a 64 SDV I owned it took me some time to figure out that the AC Speed-up control was making my car idle very fast all the time.
I have the Rochester carb which also has the TV rod connected to the carb linkage
Make sure the throttle dash pot is not causing the throttle to stay above idle. It may be rusted or not working at all.