Carbed LS swap Backfire

[RRC]

I have a stock 5.3 with a Holley carb. I have tested and researched about all I can find. I have talked to MSD and have also tried a Dayton Sensors smart spark with no luck. Everything I have done to this point has made a difference. But i still have a backfire through the exhaust at start up. Good grounds and 12v, new sensors... i have got rid of the misfire and have a good idle. I have tried different timing curves with the laptop. Im finally out of ideas so here i am looking for any help... I'm sure it's something simple..... also have good vacuum.