1987 Mazda B2200 backfires thru exhaust

I have a 1987 Mazda b2200 truck 4 cyl 2.2 5 speed.It recently started backfiring Thur the exhaust.I read that it could be a few things but did a search of and found that it could be a few things.1. dash pot found it on carburetor seems to push in and pop out OK read that it should push in slowly and pop out quickly seems to do just that.2.Timing I don't have a timing light but I did adjust the timing I marked the timing for reference then advanced and retarded timing test drove didn't stop the backfire problem reset to original position.3.I took out PVC valve and shook it sounds OK but don't know how accurate that test was either way it was on the list of possibilities but was a unlikely one.4. the exhaust is broken back by the front of the muffler could this cause this problem or was it a result of problem?5.Looked over vacuum lines seem to be OK there's lots of them but didn't see any obvious leaks.This truck runs pretty good otherwise but when I back off the gas it backfires threw the exhaust more so at hi speeds around 60 but at low speeds also at times.Any Ideas common problem with these ?Thanks in Advance

'87 and 2.2 differ, but my '82 626, running a 2.0, was backfiring like crazy anytime I came off the gas (shifts, decelerate, etc). Finally tracked it down to a bad air-injection control valve vacuum line - it had enough of a crack in it that it wasn't pulling the valve shut to stop the air-pummp from shoving air into the exhaust when I came off the throttle, which made the mix in the cat just right to go *BANG*. (Violently enough to blow the muffler wide open before I finally tracked the problem down) Replacing the vacc. line cured the problem.