[elmariachi]

You likely have an exhaust leak somewhere and its allowing air to be drawn in on deceleration, creating a lean condition and the backfire. Its definitely not normal. I would look around the header-to-head gasket for black soot, and around the header to side pipe flange. If they are slip-fits that is likely the culprit spot for sucking in air on decel. It needs to be fixed because where there's inward suction on decel, there's a damaging exhaust leak on accel.

P.S. If it were excess fuel (its not) that would be even worse because that engine should have NO unburned raw fuel leaving the exhaust, period.