striped cam ???

I have a 355 SBC with a cam duration 228 degrees at 050" .511" lift inlet and exhaust. the engine is not recenty rebuilt but has stood a while, I have had a few problems getting this engine going and gradually it has started backfiring through the carb, this backfire is in sync with the engine, not random. I have had a look around and found a large clearance on no 4 exhaust rocker and noticed that the lifter does not travel the same distance as others. My theory is that because no 4 is not exhausting the cylinder is still under slight compression when the inlet opens, causing the popping and banging back up the carb.

what are your thoughts ?