I just swapped a 390 into my 74 gran torino, I got it running and driving but the engine wasn't running the best. I parked the car this morning to try and mess around with the engine timing. I didn't do anything major to it before it kept backfiring through the carb when cranking and wouldn't start. I tried retarding the timing and it helped but it got too retarded to even start. It seems to be backfiring through only the secondaries and there's soot around the barrels. The car was driving just a day before, It was just having symptoms of being too lean. Thoughts?