if you are backfiring through the carb you may have an intake valve issue, either chipped, bent stem or not closing well. Intermittent could be due to something like a broken valve spring which would have you relying on compression to slap the valve shut. Broken or weak valve spring could also cause some return lash (excessive) in the whole rocker, push rod, lifter, cam lobe motion delivery.
dig?
Pull valve covers and twist on each valve spring with valve in it's closed position. This will be the spring's most relaxed position, it should offer some preload on the seated valve, holding it closed.
A broken one will be obvious, weak springs could be random here or there caused by the spring fatigue or possibly a sunken valve.
