This is a little long so hope you bear with me and can help. I have a 85 Camaro with a carbed 350, it is all stock but the headers and carb. and manifold that is a Edelbrock RPM performer with a 1406 carb.The carb has been on for about two years and I did not really like it because as it sat for a week or more it would act like it had no gas in it. After I got it running it would have more power and you could always feel the back two kick in. Because I did not like it I went with a holley 4bbl 600 cfm.
I always started good but did not feel like it had the power the Edelbrock had.
After a bit of driving around it would start acting like it was getting to much gas at low rpm's and would die at stops, if I would try to give it a lot of gas real fast or try to pass someone it would just die too.
It would always take a lot of cranking and time before it would start and run.
After a few times of it stopping at lights I went and put the Edelbrock back on and it is doing the same thing now.
I have a heat spacer under the carb. and on the port side I'm getting about 20 pounds of vacuum at idle.
I have chech for vacuum leaks and can find no leaks.in fact if I take th plug off of the port vacuum it idles better. From the first time I had the carb on and now I have not done anything to the motor or the carb.
What do I do now????

Have you played with the two idle mixture screws on the front of your Edelbrock? I had an off idle stumble on my Edelbrock-equipped big block (not nearly as severe as what your problem sounds like) and turning both screws 1/4 turn between testing until it smoothed out worked well for me. Those screws I think are air bleed screws, so turning them out leans it, or adds more air, like when you pull the plug off the ported vacuum.

Loading up at idle could be a float adjustment issue. Floats on a Holley are adjusted by the nut and screw at the top of the float bowls. Remove the sight plug on the side of the bowl and adjust the floats until fuel level is just at the hole. On the Edelbrock I think the float is adjusted by removing the top of the carb and bending the tab until the top of the float rests 7/16" (I think) from the underside of the top of the carb when inverted.

The puzzling thing is why would this problem surface by itself with no changes being made? And why would both carbs do the same thing? This points to perhaps the ignition system. A wise old man once said 90% of your carburetor problems are electrical, meaning look at your ignition system first to verify that it is operating properly. Another good reason to pull the spark plugs is that they will tell you a lot about your mixture. If the electrodes appear dry and dark brown you're fine. Lighter colors indicate a lean condition, black means rich, and if they appear wet and smell like gas you're way rich.

Good luck with your problem, and I'm sure smarter ones than I will have a lot more to say...

I have set the idle screws and I did pull one plug and it was a light tan and was dry. I checked the vacuum adv. and it was free and working good.I am going to try to warm it up and get to die from me here at home and pull a plug and see if it is wet or what. All wire to the dis. are in good shape.The motor has only 3000 miles on it.

If both carbs are doing the same thing I would say your problem lies elsewhere. If you pull the hose off the vacuum advance with it running can you feel vacuum with your finger on the hose? Do you have the vacuum advance hooked to ported or full manifold vacuum? Maybe you could check the timing just for kicks. Do both carbs have electric chokes? You might check to see that the choke wire didn't get pulled loose and you still have 12 volts to it. Maybe I will think of more by morning.