Rough idle- low vacuum

[Sagesurfing]

So I got a 1972 K5. It always had a bit of a rough idle. It bugged me and so eventually I swapped out the engine for a new crate 350 from Blueprint. The only parts I didn’t swap were the intake manifold, the carburetor, and the distributor. But the new engine still had a sloppy idle. The kind that is slightly inconsistent, that throws the timing light off every so many bleeps. Anyway… since then I’ve swapped the carburetor (for another same Edelbrock 1406,) swapped out the distributor, taken out and reinstalled the intake manifold, studied the carb in depth and learned everything about tuning it, and hunted down every possible place I can think of for vacuum leaks (around the intake manifold, base of carb, surface of intake, valve covers, disconnected any and all hoses and lines including break and transmission,) and yet… rough idle and vacuum at 15 jumping around a bit as it sputters. When I accelerate it sounds and runs great. But to keep it from dieseling I have to run out the air mixture screws 2.5 turns. I set my idle rpm at 800. What else could it be?

[geezer#99]

Lack of enough initial timing!
What’s your timing set at?

Set your idle at 700 for checking and with vac advance plugged off.

[Sagesurfing]

Initial timing set at 15 advance. I don’t use vacuum advance per engine manufacturer recs so it’s plugged

[geezer#99]

Yup!
I’ve seen that before.

Plugging in your vac advance for testing won’t hurt a thing. Nobody will know.
Give it a shot.

[RustyPile]

An 800 RPM base idle setting will cause run on (dieseling) when the ignition is turned off.. Unless you have a cam that has a lot of overlap, the idle speed should be about 600 - 650 at the most..

The air fuel mixture screws are for adjusting the idle mixtures not idle RPM.. You seem to be concentrating on the search for vacuum leaks.. There are a million things that will cause rough idle conditions.. These are only a few. Spark plugs fouling from overly rich condition. Faulty distributor cap, rotor, and/or plug wires.. .. Not to mention improperly adjusted valves and/or timing.. You will have a difficult time obtaining a suitable tune with not having vacuum control advance on the street. Never mind what the "specs" are.. Give the engine some settings that it likes...

You may not like the following: I'm just not a fan of Edelbrock carburetors. They, too often, cause the very problems you're having..