A few questions

Let me double check what you are calling backfiring. Are you saying that it is firing from the carburetor or backfiring through the exhaust?

If it is from the carburetor you are seeing the exhaust and intake open at the same time. There are several things to start with. First is your timing with 8-14 degrees before top dead center? Also check the valve spacing intake .0010 and .0020 on exhaust. While under the valve cover check and make sure there is no play in any of the push rods. If you see any you will need to replace the seats.