Good luck with that! Carbs that have been modified by a tuner can be really tough to sort out since you never know what they did. Sounds like you're flying blind without any diagnostics which makes the job nearly impossible. You really need some data to help solve the problem. EGT, header temps, AFR, etc.

You can adjust idle mixture screws to see if they work. That will at least give you one clue without too much work. If you haven't already you can also adjust timing at idle to see if that does anything. Since this problem is at idle I don't think I'd worry about the power valves too much. Look down the throttle bores while the engine is idling and see what is happening. See if there if fuel being pulled out of the boosters and if so which ones. That should be a four corner idle carb so try all four idle screws.

Do you have anyone who can help? I have a feeling that someone with some race engine knowledge would be a big help.