Base ignition timing is controlled by the computer.
There are about 89 things that could be causing your problem. Here's where to start...
1. Fix that ground wire! And verify all others. Engine grounds are critical, especially with the RENIX vintage.
2. Tuneup. Do NOT underestimate the importance of having a solid secondary ignition. Fresh Champion copper plugs gapped to .035, quality plug wires, distributor cap and rotor and a new fuel filter.
3. Verify your fuel pressure. Backfiring is usually caused by an "imbalance" of fuel/air ratio. For your 1990, you should have 31 psi of pressure at idle. MUST be verified with a fuel gauge; many parts stores will rent you a gauge very inexpensively; call around. Then disconnect the vacuum line leading to the fuel pressure regulator (be sure that line is pulling good vacuum and is not wet with fuel which would indicate a ruptured regulator). With that line disconnected, pressure should jump to approximately 39 psi.
4. Test your MAP sensor. And verify the vacuum line leading to that sensor as well.
You have to approach this in a "simple to more complex" strategy. Keep us posted!
99 Cherokee, 4.0 AW4, NP242
Past Jeeps: 49 Willys, 81 Scrambler, 88 Comanche
Without "data", all you have is an opinion!