I have a 72 Duster with a 360 4 barrel, totally stock except for a 340 cam. I had an Edelbrock 600 VS that had a high idle problem from the moment I put it on. I had no experience rebuilding an Eddy carb, so I had a local shop go through it. The choke was never hooked up. No external vacuum leaks were found and it made decent power. No tach, so I cannot say exactly how fast the idle was , but it ran at a rate higher than most carbureted cars do when the choke is functioning during warm up. The idle set screw was off the seat. Timing was at 10 degrees btdc. No engine smoke. I tried another carb, this time a 440 spec Thermo Quad. I get the same thing with this carb... fast idle that I can't seem to adjust down. I have played with the idle mixture screws and the timing, at separate times. Neither made a difference. The base gasket is NOT interfering with the throttle blades, and all vacuum ports on both carbs are either hooked up or plugged. It is a factory iron intake, so I cant test any of my Holley carbs. I am tempted to swap on a Performer 318/360 so I can switch carbs, since it has the dual pattern. Any ideas ? I would prefer to keep the Edelbrock carb since it ran so strong .

It sounds like you have a vacuum leak someplace. Intake?

You need to do a proper tune-up, which includes setting the carb, ignition timing, and to make CERTAIN all of the components not only work, but are working in conjunction with each other. One little change in one item will affect all the rest.

You also checked the throttle cable, that it's adjusted correctly to not hold throttle open, yes?

is it only the fast idle that is too high? the curb idle is OK? WHEN did the prob start as in after any part changes. A flat plate from edelbrock at your parts store (it's cheap) that carries speed equip would let you try out your square bore holleys on the oe spreadbore intake.

make sure the eddy spring tension is not stuck on the throttle shafts

make sure it not hitting/hanging up on the intake at the carb base

I had trouble with one hitting the choke well and had to use a 1" spacer to lift the carb so linkage would clear

what he said on throttle cable

unhook it and try

the choke linkage and fast idle cam is out of wack maybe

manual curb idle screw is not in to far correct?

seems odd 2 diff style carbs do the same thing,make me think its something mech or a vac leak

pcv hose tight,missing a cap on vac port???

plug in the back for the brake vac port

brake vac booster good and not leaking??

I have ran both carburetors with and without the throttle cabe connected, and they both idled high. Both chokes were inoperable. I have sprayed ether around the bases of both with the engine running and niether one showed evidence of a vacuum leak. The intake was removed for a cam swap last year, Fel-Pro gaskets , torque sequence...the whole bit.
I know that it is possible to have 2 defective carbs in a row. I used to work for a major auto parts chain...I have seen it happen. My skills with carburetors are limited, which is why I wrote this post. Any more suggestions?

If there are power brakes, you could have a vacuum leak at the power brake booster or at least the line going to it. How about a vacuum leak going to your distributor vacuum advance unit?
Try capping off the fittings going to both of these and see what happens.

It has manual disc, so no booster. I'll try the distributor vac can hose. Who knows where the problem lies. I'm willing to try almost any idea that is free.

Vacuum advance connected to the proper port? Throttle blades adjusted properly? Kickdown linkage adjusted properly (not pulling throttle open?) No vacuum leaks at manifold/head intake port areas?

Your vaccum advance isn't hooked up to manifold vacuum?

I did that once and had a similar problem.

Thank you to everyone so far. It is a 4 speed trans, so KD linkage doesn't apply. The distributor vac hose can be connected anywhere and it does not matter. I backed the timing waaaaay off to get the idle down, but I know that 2 deg AFTER TDC isn't good. I am lucky that I have beed a junkyard rat for so long, as I have a stash of extra parts to test.