67Dart273 said

If you never have, I would just do a through and complete tuneup

1...Do a compression test and leakdown test. I don't remember if the 74's have adjustable valves if so check them
2...Examine all ignition components condition, wires, cap, rotor, etc, and set timing. Don't set it to "stock" which is likely TDC. Set it for at least 5BTC
3...I agree that the idle problem is likely a carb "getting" (gotten) dirty. You might get lucky. Sometimes you can pull the idle screw, blow some cleaner through it, then reset. Pull it again idling, work the throttle and hold your finger over the screw hole. This will suck air through and also help clean it

Set timing BEFORE you adjust carb. Set carb fully warmed. Make CERTAIN you have not lost track of any vacuum hoses and let them open. Adjust mixture for highest vacuum or idle speed, and "finish" with screw "just leaning" towards lean (CW) side. Reajust idle speed and mixture at last

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AJ/FormS said

This is a common symptom of the throttle blades being too far closed and/or the mixture screws set too lean. This is often a by-product of too much Idle-Timing.
Altho the PCV could be faulty.

EDIT
Another possibility is a wrong "wet fuel level" in the bowl; likely low, but could be high as well.or a contaminant in the fuel, but this is less likely.
In any case, your carburator circuits are all designed to function, around a specific "wet" fuel level. The spec in "the Book" is a "dry" adjustment, designed to achieve that specific "wet" level. The spec is usually pretty accurate.
Whenever working on a cantankerous carb, this is always the first go-to.

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