Carb issue?

Currently i have low unstable vacuum at idle of around 5-8". The carb will not allow me to adjust idle screw low enough for the idle circuit. This is causing a very rich idle issue. If i hold the secondaries open a little rpms jump and vacuum goes to a solid 10"-12" depending on rpm

So its 5-8" of vacuum and rich and when you introduce more air it leans out and produces 10-12" more vacuum.

At what rpms are differences?