rough in gear idle and off idle stumble

OK, equipped parts....

225
pushbutton 904
holley 1920 with new accellerator pump
points ignition set to 10* points gapped correctly
valvetrain is 'quiet' (checked it cold when the engine was apart and on the stand)

Issue is backfiring and stumbling if you 'load' the engine suddenly and the idle mixture screw does nothing to improve anything as well as changing the timing. Once you are over 2000RPM or so it smooths out and runs fine.

any suggestions guys?