Engine runs at a higher RPM but doesn't idle - possible causes?

So, I decided to put a completely new carburetor on my 383 instead of trying to repair my junk original Carter 2 barrel. I got the same model, albeit from a 1969 model, but it's still pretty much the exact same type of carburetor and bolted right on. It was an ACDelco rebuild, I adjusted the idle screws to 1 and a 1/2 turns and tried to adjust it: it doesn't want to run at all at the idle RPM, but if I rev it it fairs a bit better.

I've heard several suggestions from elsewhere and on this forum (many thanks to CBODY67), ranging from a vacuum leak from the carburetor gasket, to a timing problem, to an incorrectly adjusted or clogged carburetor.

On this carburetor there's two vacuum lines, one for the distributor advance and another on the very back. I didn't cap these although I read later that you should, so I'll take that into consideration for next time.

It is worth noting that my original carburetor, while having an issue with acceleration, actually handled idle well, which is why I'm not sure if this is a timing related problem. I have a thick style gasket for the carb which I could put on instead if that would make a difference, I just want to go from the least intensive task to most intensive. Any thoughts on this situation?
it sounds like a carb issue to me spraw carb cleaner around the intake & the botto on the cassb if ot runs better u hace a vaccume lran . have u snuffed the carb? did it die then the carbs on if it runs better u have a internal carb vacuum leak
 
2 of your photos show the large vacuum line at the rear of the carb isn't hooked up, did you correct that already?
 
When I ran it yes, all of the vacuum ports were capped and the choke was hooked up along with PCV - still has the rough idle though.

When I first got the carb I only inspected the outside, but I didn't see any major areas that could be an issue with air leakage - didn't see any cracks in the body, gaskets looked alright, choke line looked alright - I didn't take it apart (which I guess I'll have to now anyways), and I'm not sure if it's the throttle mount that's the issue here, since I've taken off my old carb before and never had a vacuum leak issue from that - I guess the carb gaskets were misleading?

Although the only clue I have here is that the hissing is louder on one side than the other - passenger side.
 
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