The car is a 84 z28 with a 91 chevy 350 and a summit carb and manifold, i had it running this morning and i noticed some gasoline leaking from the floater gaskets so i replaced them and when trying to start it again it started doing this, when the loud sound happens white smoke comes out the carb . Is it a form of backfiring? I changed nothing except the gaskets for the carb floaters. This is the video please give your thoughts on what this could be, thanks!

 

Sounds and looks like you are 180' out of timing... Swap your wires around on the cap till they are completely opposite sides of where they were and try again, from there you can fine tune till you get it right.