blows muffler off! on 1992 Cadillac DeVille

Set timing according to chiltons book,10 degrees BTDC, computer advances up to 46 degrees according to OTC scan tool, altenator tested good at parts store. New plugs, wires, cap and rotor. Port fuel injection. Intermittent backfire and it blows the muffler off. 4.9L, runs good except for the backfire.

Asked by for the 1992 Cadillac DeVille