A car backfire can be caused by alot of thing. Because you have the dual exhaust is most likely why it is happening.

You dont have enough back pressure so the exhaust valves are floating (meaning they arent closing all the way) the exhaust will warp the valves, even if they close all the way they wont completely seal.

When the piston compresses the air, and the fuel is injected into the cylinder, some of the fuel/air mixture escapes out of the un-sealed exhaust valve. When the spark for that cylinder fires, it ignites the gas inside your exhaust.

To gain back pressure take off the dual exhaust, you dont need em', if you leave it the way it is you will lose all compression in that cylindar(s)

Hope this helps