BACKFIRING 1987 STATIONWAGON 1800 OHC 4WD

I just bought a 1987 STATIONWAGON 4WD 1800 OHC MANUAL TRANS with 419,000km (about 250,000 miles) got it cheap all pretty good except for slight misfire and backfiring when changing gear. Replaced coil, dizzy cap, rotor, plugs and tested leads all good now runs well no misfire but still backfires. Dizzy needed max turning to get to 10 degree BTDC spec timing otherwise was way to advanced, suspect one of the timing belts one tooth out of specs, is this likely cause and will it cause backfiring? Also it has catalatic convertor in exhaust system but inside has disentegrated to nothing due to age leaving pretty much a tin can inline of exh system.I think this is contributing as well as the excessively emissioned carby set up which iam thinking of replacing with basic type from older model 1800 OHV eng, any ideas, help thoughts much appreciated :)