Need help with backfiring Comanche

My friend has a 90 4.0l comanche and we really need to figure out why it is backfiring. Its wierd and weve been narrowing down problems for days. First it was backfiring consistently until the jeep was driven home. After being home for a day, we drained the gas tank empty by disconnecting the fuel line from the fuel filter and cranking it until the remainder of the gas pumped out. The gas looked pretty bad so we put a fresh 4 gallons or so in the tank along with a bottle of dry gas and lucas fuel system cleaner. Right after that, the jeep started up, backfired 2 or so times and ran at idle for 3 or so minutes without backfiring. We called it a night. Today, after starting the jeep up, it began backfiring again. Changed the fouled plugs and it backfired still. While changing the plugs we checked the engine compression. It was around 90 in 3 cylinders and about 115-120 in the other 3. Again, this is a 90 4.0 litre with 130,000 miles. Is that too low and would it have any affect on the backfiring? The wierd thing is, it will only backfire at idle- open the throttle some or all and it runs pretty good, just a little rough but NO backfire. SO, what are the possible causes? The injectors were pulled with the fuel rail and we cranked the engine over. The injectors all squirted in their sequence and looked pretty good when we cranked the engine. None appeared to be sticking. Still havent checked the fuel pressure. Also, its possible that the ignition coil may have gone bad. Would that cause a backfire at idle? We hooked a spark plug up to the ignition coil wire and held it to the engine block. There was an orange spark coming out of the plug but not a blue one like some have said. There was carbon on the rotor button and distributor cap. We scraped it almost all off and it still backfired. Any concern you think?

What we really need to know is what are the probable causes now and what does it only backfiring at idle mean? Are there ANY SENSORS in the renix that could cause a backfire if bad? Could the gas still be bad after sitting and settling for a day? Like I said, we only pumped the gas out of the tank, but did not pull the tank. Does water settle in the gas or rest on top of it? Thanks for any help or suggestions. It is a pain to diagnose this problem and it would be great to get this running good again.
-sean