Backfiring issue

Well I would start with the simple diagnostics... If you don't have one a compression gauge is probably less than 30 bucks... Pull all the plugs and run a compression check. If you have good compression go from there. If you don't time to pull the engine...
Then new plugs new wires new rotor new cap and I assume it has an electronic ignition?.. timing timing timing everything comes from timing.... Make sure you don't have any leaky spark plug wires, ones that are hitting the exhaust or worn out and letting spark get out... A lot of times it can be done at night just running the car and you'll see the spark arcing...
While you're doing your compression check I would take the valve covers off while it's turning over and see if I can see anything "NOT" moving or not moving enough like a dead cam. Check for broken valve springs and such but a compression check should tell you A lot of that... Again keep it simple and look for the obvious stuff... Not time to start spending too much money yet...