Timing question 318 with LD4B

Timing can be anywhere from 5 to 25 and by itself will NOT cause backfires thru the carb during a cold start or idling.

A backfire implies that there is fire in the intake manifold.
More often than not, this happens because of an incorrect firing order, cross-firing inside the cap, induction firing along the wires, a wet coil top, or just plain old lean mixtures, due possibly to a non-functional choke, but could be a huge intake leak like a blown PCV hose or bad brake-booster. And don't forget stale gas and/or water in the gas.
These are your first go-tos; the simple stuff.
But this can also happen when one or more intake valves are not closing. This is usually because the Cam is out of time, or valves are hanging up, or are burned.
Therefore
after checking the simple stuff, it is time for a timing-chain stretch test, then a check to see where the rotor is in relation to where it is supposed to be, then a compression test, and finally, possible a check of the cam timing, or at least to see where split overlap is.

If you have log manifolds, and a restrictive single exhaust or a plugged muffler/cat, I suppose a pressure build up could backfeed into a cylinder with an open exhaust, on the overlap cycle, but that's a stretch.