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I'm not a smog expert, BUT it is clear that when you raised the idle you went sufficiently off the idle circuit and into the cruise circuit, indicating that the problem is in the carb idle circuit



I agree there. I also sprayed some carb cleaner down alongside the metering rod passages.
It may be safe to say that a quality rebuild of this carburetor would improve matters without having to resort to any tricks. I've rebuilt Holleys, but these Carter T-quads are a wee bit more complicated!