while your doing what mrfixit suggested also check intake and exhaust manifold gaskets for vac leaks or any vacuum lines that could be leaking.
If the backfiring is consistant then it may be a cracked dist cap or something up that alley but i wouldn't rule out a vac leak at one of the manifold gaskets.
if you have time pull a compression test after you check the valve adjustments. If any cylinders are low you could have a cracked exhaust valve.