350 running bad----need help!

[Farmstore34]

Hey all,
I need some help on a problem with my 72 K20. It has an older crate 350 with an edelbrock intake, carb, and mild cam. It has always run and idled fine. All of a sudden it started running poorly and will hardly idle. It runs fine under acceleration and above 1800rpm but below that it acts like its running on 5 cylinders and idles erratically (seems like its running very lean). I changed the fuel filter, then cleaned out the carb but no luck. I then swapped in another Edelbrock carb that was working fine and that made no difference. I suspect a vacuum leak, but have found nothing around the carb, no bad hoses.
I have a feeling it may have been over revved on the day this started, so I'm not sure where to start. I dont hear any knocking or ticking, and other than at idle it runs fine. Any ideas?
Thanks!
Colby.