[johnsongrass1]

Id blame an exhaust leak

 

[johnsongrass1]

Open the idle mix screws another 1/8 and tell us what happens

 

[johnsongrass1]

There are two things that cause exhaust backfires and that's retarded timing and air leaks.
When you have unburned A/F in the exhaust, once it gets pushed out into the header and meets the oxygen in there it'll self ignite in the tubes and go bang.
Either make it so it will completely burn in the cylinder OR close off the exhaust so it can't burn in the tube.
Check your ignition closely.

 

[johnsongrass1]

Your vac at best might make 10psi, the engine can make a few hundred on blow down.
Ignition, New, doesn't always mean good.

 

[johnsongrass1]

Add 2 more degrees of total timing and go down 2 numbers of jets as well as turn the mix screws in 1/8 turn.