My 69 Dart slant was like that for decades. I think it was always idling lean. A 4th carburetor (Holley 1920) fixed it. Another symptom was that when trying to optimize the idle spark advance, it liked a lot more advance. Increasing to 10 deg BTDC and the engine rpm would increase, increasing advance even more. I think it needs more advance when lean. If ever doing exhaust work, weld an O2 sensor bung in. You can get inexpensive rich/lean indicators, or go w/ a wideband monitor for more fidelity.
