Need some help with diagnosis

I've got a '73 duster 318 2bbl,I was driving and stopped @ a light.when i accelerated it started backfiring thru carb! I started checking things and am stumped on what happened.I looked inside dist. cap Looked good,ckd.for broken valve springs Ok,took compression ck. 140 lbs ea cyl. Now it still runs but when i rev engine above Idle it backfires thru carb ??I need some ideas on what Next !