How lean is too lean?

7milesout said

So, what I'm going to go home and do is go to the next stiffer metering rod springs. As I recall, I think I'm on the middle springs. What should happen is, as my foot opens the butterfly more, and air can flow in easier, the more open butterfly (and additional air flow) reduces the vacuum, which is what the metering rod springs are "fighting" against. So with the next stiffer spring, those springs should tend to "win the fight more" against the reduction in vacuum. And when the springs win, they pull the metering rods more out of the jets, allowing more fuel to flow to match the increased air.

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You're on the right track... Carter used to offer strip kits for this reason, but since Edelbrock took over, they've cornered/took over the market on those. seems The Carb Shop has filled the void since then.. I used to work with a lot of carter stuff, back in the day when you could still find them in salvage yards. I've moved on to Holley's since.
The strip kits had several different springs in the kit, and even had a selection of shims to 'fine tune' if needed. (if I remember correctly..)
The Thermoquads had a small screw in the middle of the metering rod bar, that rode on a eccentric in the middle of the throttle shaft, to adjust metering rod 'ride height'. This made all the difference in the world in such conditions you're describing. unfortunately, the afb/avs doesn't have that...