My experiences are that all of the vehicles I have had do NOT have a glass-smooth idle (like any FE Ford V-8). There is always a little bit of variation, like 10-20rpm on the dwell tach. Whether it was a Stromberg WWC3, Holley 2210, Rochester 2bbl, Carter BBD, Carter AFB, or Carter AVS. With finesse, I could decrease the variations, but they were still there.
"Chugging" can be a different situation than just a "slightly rough" idle, though.
Vacuum leaks? Unless it is from a broken vac hose (hvac system under the hood, cruise control servo, EGR valve supply, etc., it would be more probably from loose carb mounting nuts, from my experience.
Vac leaks from the power brake booster usually "present" when the brake pedal is depressed and the engine starts to miss. But the miss goes away when the pedal is released.
Intake manifold vac leaks are in all of the trouble trees as a potential possibility. Always mentioned, but probably not with a cast iron intake and the normal Chrysler B/RB style of intake manifold gasket.
So, first check ALL of the bolt torques for the intake manifold bolts and the nuts on the carb baseplate for tightness. Factory specs/orientations only.
Then check all of the underhood vac hoses for stiffness and related cracks by gently moving them. Hairline cracks can develop in the hoses just as they can in the plastic parts they might attach to (vac trees and such).
As for "detection fluids", gentle spritzs of spray carb cleaner has worked for me. There are other flammable fluids used with car, or even water can be used. Whatever works.
"Sticking needle and seats"? Always mentioned in the trouble trees, but unless the carb is visibly flooding, not an issue as to idle quality, from what I have seen. Just my experiences and observations.
End result, much effort can be expended with zilch results!
From my experiences with the Holley OEM Replacement 4175s, most probably not your problem either. As long as it is working well.
Have you changed ALL of the rubber fuel lines on the car, tank to carburetor?
There is only so much a tach will tell you, so I would suggest you find a repair shop with a diagnostic machine with an ignition oscilloscope on it. Or a separate oscilloscope / tach combination so you can look at EACH spark plug firing event, in real time. Only THEN can you start to pinpoint issues related to the ignition system and how they might correlate to the idle speed/quality issues.
In our current computerized world, finding such a shop and an employee who knows what the traces are telling him might take a while. Might even take soem "road time", too?
In the mean time, put some P.E.A.-containing fuel system cleaner in the tank and put the car on the Interstate at 70mph for at least an hour or so to get the fuel system and combustion chambers cleaner. Repeat until that tank of fuel is depleted.
Ensure that all of the spark plugs are gapped EXACTLY the same and that all plug wires are in good condition. No need to buy new ones unless the old ones are causing issues.
Hope this might help. Just MY experiences and observations, over the decades. YMMV
CBODY67
