The backfiring makes me think timing problem, like a bad pickup coil or something. Once again I suggest cranking it over and with compression tester make sure the spark is happening near TDC. Also, if possible, make sure only one plug is firing at a time. 4 plugs grounded to the chassis should confirm this. You may very well have a defective TCI.
Also, for S & G put in a new fuel filter, they are cheap and it's a good idea anyway...