850 thermo quad

I have seen the idle hang up when the throttle valves are too far up the transfer ports.
Reduce your curb idle speed, and if necessary increase your idle-timing slightly.
This assumes, no vacuum leaks, and the choke is all the way off, and the secondaries are fully closed but not sticking, and the PCV is correct and functioning, and the charcoal canister is correctly plumbed and inactive at idle, as it should be, and the signal line is not sucking air. lol