If you have a Holley 4150 type carb, then the transfer slot needs to be ballpark correct first.
Then the mix screws are for fine tuning the slow idle.
On a really radical engine its usually neccessary to bypass some air to get the idle throttle position. Old school way is to drill holes in the throttle plates. I've done that and still think its fine to do so. But there are other methods used that are clever. One is to crack the secondaries. Another is Bill Jones method one company calls "Idle-eze" but you can do it yourself. Another method is to use the PCV valve.
I can't answer for Phreakish but non-smogged engines idled a lot richer than most people think.
I'll quote Tuner here:
I suppose it sounds crazy nowadays, but before smog tuning started in the mid '60s 5% to 6% CO was a normal idle adjustment for slowest smooth idle.
If we look at the relationship of AFR to CO, 12:1 is just under 6%, and 13:1 is about 4.5% CO.
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That's good for our understanding, but the WBO2 is not to be fully trusted. Especially at idle and especially when combustion is not doing standard predictable things. I agree with "don't tune for a number" Our real goal is to make as good of power as we can at idle rpm.
I think the reason engines need rich mix at idle is that they make very little power. When we put the selector in gear or slip the clutch without touching the throttle, it easily could be close to maximum the engine can produce at that rpm. In fact some engines can't go into gear without stalling unless we open the throttle.
Might be worth looking at some text book info about AFR needs.
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