Tuning with jets, Power valves and an O2 guage

The engine stumbles and dies at idle when I turn the idle mixture screws in all the way. I only did that as a test since I was actually able to get the A/F ratio in the proper range with most combinations of jetting & PVs so far.
I have tried setting the timing to 24-25 initial. Idle vacuum only went up to about 11.0 compared to 10.0 with the timing set to 19 initial.
Can somewone tell me what it means when there is a large RPM drop from idle to in gear?