Need 65 Caddy Specialist for a Rough Idle Problem

My Origianl 65 Caddy Fleetwood Brougham with 56000 Miles has had a rough idle since I purchased it 6 years ago. It has the Rochester 4GC Carb and an "open PCV" system.
So for this has bee driving me up the wall.
I have rebuilt the original Rochester 4GC Carb 3 times with 3 different kits, with a little benefit to the rough idle. Problem is the kits are the "One Size Fits All" Rochester 4GC and has many gaskets that are for specific applications. I used the Throttle Body to Base gasket like the one that came off, but I don't konw if the carb was previously rebuilt.
The Barb Base to Throttle Body Gasket has some passsages called" Vapor Vent Passages,' both on the primary and secondary bores. Other gaskets did not have these on the primaries, but they would not fit.
There was still the rough idle.
I then changed the plugs, wires, dizzy cap and rotor, condesor, points, checked the dwell and timing. All seems to be up to snuff.
Still no difference.
So I got a Crane Fireball XR1 Ignition Module, which fits in place of the points and condensor, automatically setting dwell. No difference when idling.
Covered ALL the vacuum lines leading to the carb and the intake manifold. No difference either. HUMMMMMM
Bought a Professionally rebuilt Rochester 4GC, which made a slight difference.
I am at my wits end with this idle problem!!!
I know that the Choke Heat Tube that runs through the Intake manifold is rusted throught, but have been told that it wouldn't have any influence on the idle. I am not sure if they are correct on this as they aren't a mechanic.
So I don't know if that is true or not.
Also, I think the rebuilt carb has the vapor vent passages on the primary bores as well as the secondaries.
I am hoping to find someone who really knows 65 Caddilacs and/or Rochester 4GC Carbs to help me out with this situation, as I would like my car to idle like a Caddy should. Nice and smooth.
Anyone able to offer any help???
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Owned one, anyway rough idle is a broad issue. How is the vacum steady and around 18-20? Done a compression test? Why do you insist on blaming the carb, done the idle mixture screw adjustments on car, hot. The engine should get real rough as the screws are turned in, then back out till the idle speed is the highest on each idle screw adjs. 40 year old engine here, how rough is rough?

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It's been a long time since I worked on one, but I recall that there are gaskets between the carb and the spacer block and then again between the spacer block and the intake manifold. If the right things aren't in the right place the engine will run badly.
Do you have an original Cadillac shop manual and/or parts book for reference? If not, get yourself a set. Reproductions are pretty readily available from the specialists.
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You were wise to offwer this bit. The choke heater tube does in fact have vacuum applied to it to pull the heat from the manifold as it heats up. By all means, repair this sub-system, as it helps with cold starts plus draws vacuum if rusted thru. This should help. On another train of thought, spray a burnable liquid(NOT explodable, like gasoline) like WD-40 around the base gasket while idling; also spray around any suspected vacuum-leak points: intake mating surfaces, vacuum hoses/fittings going to power brakes, etc. If engine suddenly changes rpm's you've found a vac. leak--fix all of them, and carb should 'do all it can do'. Also, check for vac leak at vacuum-advance on distributor. If it is open, replace with a good one--engine needs this for idle as well as acceleration. Also check trans modulator: it should hold a vacuum when applied to hose going to it. Luck, s
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Also check PCV valve. If it is dirty, idle will be rough. It is easy to replace. I know the Caddy engines can idle smooth, I owned a '62 with the 4GC carb, and a 67 with a quadrajet. I could balance a nickel on the air filter at idle with both engines. Roy

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