The best place for a Holley [ or clone ] is to never remove it from the box it comes in.....
Have you checked the usual suspects:
- flooding
- float level
- choke staying on [ if equipped ]
- do the idle mixture screws adjust the idle. Screwing them IN should kill the engine. If not, the engine is getting extra fuel from somewhere, most likely from t/blades open too far exposing excessive T slot [ 0.040" or less of T slot should be showing at idle ].
You say 'mild' SBC. Cam? Cu in? Intake?