is it backfiring through the exhaust or intake? could be running to rich/ or lean out of many reasons (bad fuel pressure reg, intake or exhaust valve, timing to advanced or retarded, bad igniter or coil, valve clearance too tight, vac leak, egr valve stuck) what do the plugs look like? Does it only happen under load? U can set timing by checking for marks on timing belt cover falling in between marks on crank pulley or on some older cars pulling out the inspection plug on side of auto trans. Engine has to be warmed up at idle for doing this. Also check cam timing.