low compression and blowing back thru carb

15.5 Briggs only has 50 psi compression and when cranking it blows back thru carb. Wont even fire on starting fluid, just backfires thru carb. Does this sound like bad valves??? Or could low compression, be pushing past rings and blowing out thru carb. Where to start on this? Any problem areas on this engine to look for?
thanks Rick