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...
