Re: 98 jeep wrangler 4.0 backfire
I'd start simple.
One step at a time -
First check the timing at idle with a timing light. If it's close that's not it. If the flywheel trigger was very far off, it would be firing way off - the timing marks are still on 0 no matter which engine. Unless you also switched vibration dampers.
Second - to check if it's too lean - (hot manifold can get hot from being lean - it misfires, the unburned fuel burns in the exhaust) - do a searh here for "propane trick." Make the necessary mods to a propane valve - as original you won't get enough propane through the valve.
Idle - add propane to see if it runs much better. Propane will burn just like gasoline under these circumstances.
Usually too rich won't backfire or stumble - and you'd be seeing clouds of black smoke too if it was rich enough to backfire.
DO NOT try pouring a little gas down or a spraying in carb cleaner like Gumout, or ether - DON'T TRY IT - not with the backfiring - you could get badly hurt or burn the project down. Propane is relatively safe, but still -- keep your face away!
Third - drop the exhaust down - disconnect the headpipe at the manifold. After you run it, put something up in the manifold ends to keep cold air off the valves to keep the hot valves from bending. Wet rags works fine.
I'd not blame the electronics - yet. The basics of compression, spark, mixture are still valid no matter what electronics are used.
Since it runs it's not likely the computer, possible, but doubtful.
Since it runs, it cannot be 180 out, it'd bacfire but cannot actually run like that. But it still could have a couple of plug wires mixed up -- sometimes when we check we do the same mistake over and over.
Let us know how it fares.