I pulled a 600 cfm carb off my caddy with a SBC, dropped it on and the car runs great! No backfire, no pop, great throttle response.

This is an older model with manual choke and single fuel inlet. It started right up, and idled great. revved it and no backfire, took it for a drive and it drove great.

Why does the carb I took off a Holley Avenger 870, run like poo? Power valve was fine, no vacuum leaks i could find.

I started the car on the avenger, revved it, and it backfired. Swapped the carbs started it, revved it and no back fire.

No changes, reused the carb base gasket, NO tuning changes, same timing. WHAT THE HECK!!!! THIS WAS A BRAND NEW HOLLEY!?!?!?!?!?!?