Edelbrock is a good carb, but you need to tune it correctly, and it has to be tuned properly for each engine. Just because it runs well on one 350 engine, does not mean it will run on a different one without tuning it for that particular engine.
If you simply have the mixture screws arbitrarily turned out 2.5 turns, just guessing or hoping that its right, I'll bet you that is where at least part of your problem is. The mixture is likely too rich, and therefore you had to turn up the idle speed just to get it to run at all.
download the PDF owner's manual for your carb. Take the time to read it and understand it, then you can learn how to correctly adjust the idle mixture, and also tune all other aspects of your carb. Just follow the instructions step by step. http://www.edelbrock.com/automotive/...ers-manual.pdf
The Edelbrock Performer Series carburetor has conventional Idle Mixture Screws (IMS) that provide a leaner A/F when turned clockwise and richer A/F when turned counter clockwise. The idle air flow is controlled by a conventional screw that opens the Primary Throttles. The following procedure should be used to set the idle mixture and speeds.
￼￼￼￼￼1. Fully warm engine and ensure choke is fully open.
2. Air cleaner in place.
3. Set desired speed with the air screw.
4. Adjust the IMS on ONE side to get the maximum possible RPM. Do not go rich beyond the maximum speed point.
5. If the above changed the idle speed more than 40RPM, then readjust the speed.
6. Adjust the side OPPOSITE of that in Step 4 to get maximum RPM.
7. Reset the speed.
8. Carefully trim each IMS to again get the maximum idle RPM.
9. Go leaner just enough to get a 20 RPM drop in speed.
10. Reset the speed to the desired RPM.
11. This is a Lean-Best Idle Set. Setting richer than this will not improve idle quality or performance, but could tend to foul plugs."
For example, I just swapped on a 600 cfm edelbrock carb onto my 350 this last weekend. I set the idle mixture and it only required about 1&1/4 turns out. Every engine will be different.
Note the mention of fouled plugs if the idle was too rich. Pull your plugs and clean them, then put them back.
Another thing to look for is VACUUM LEAKS!!
If you have vacuum leaks anywhere, that can mess up all kinds of things, including making idle idle roughly, or not at all.
Then get yourself a quality timing light, and make sure your timing is correct, including checking your vacuum advance to make sure that is working properly and hooked up to the correct port on the carb.