Help! Idle speed screw not working :(

Yes, we just went through the same thing with a used carb. The secondary idle stop screw was CLOSED too far to the point that that secondary throttle plates were touching the sides of the bores. In that case, the carb is not allowing in enough air and the primary throttle was open too far to compensate. But when this happens, then the primaries are open too far, and the primary transition slots are exposed and that makes the carb too rich. The primary idle mixture screws become unable to compensate.

And if the secondary idle stop screw is too far OPEN, then you cannot bring the idle down properly.

1) Look at the secondary throttle plate position; they should be opened just a bit and exposing the bottom edge of the small vertical slots up in the sides of the throttle body. Adjust the small screw shown above to just expose the bottom of these slots.
2) Reinstall and readjust idle mixture with a vacuum gauge connected to the manifold vacuum for peak reading.
3) Adjust curb idle speed.

The choke has to be off and engine warmed up before the curb idle screw can be adjusted.

You may have to do a couple of iterations of adjusting on the secondary idles stop screw.

The backfires may be due to not being warmed up or timing may be off. Have you set the ignition timing to the 10-15 degree range when at lowest idle. That will get you in the ballpark to get started.

BTW, what engine, cam, etc.?