Yes, you are correct, although in the late 70s they even fixed the idle mixture screws so they had little effect even with the limiters popped off. Prior to that every "jack legged" mechanic went right to the mixture screws instead of checking idle speed, plug wires, etc. What usually happened is the OEMs increased the idle speed to as high as 750 rpm to pass the EPA tests in effect at that time. Then the customers would complain because they got a loud clunk when they put the car in gear. So they would take it to a mechanic and he would lower the idle speed. Well then the car idled like poop so off came the idle mixture limiter caps. etc. etc. etc.

 

Timing comes into play, or ignition advance changes A/F drastically correct?

One thing I don't understand, though it doesn't necessarily apply to these particular beasts, is why ignition timing that is retarded reduces hydrocarbons at idle. 70s smog era rigs, the manufacturers went through all these contortions and contraptions to detune the engine, and then burn all the waste after the fact. Apparently a well running, advanced timing engine has low HC, but a high level of NOX, which contributes to the reddish smog seen over cities in the past or where people still have jobs.

This makes no sense to me in the first place because "slow" timing means unburned fuel is being lost out the tailpipe. It's one reason why manifolds will start to glow, although excessively lean mixtures can melt things as well.