So I replaced all the Jets with the forum recommended ones, as well as peirce manifolds agreed with the recommendation. I put on a new intake/exhaust gasket and tested, no leaks. Absolutely no change in the back fire. It still only does it after it is warm. This is driving me crazy! if it were a burbly pop, I could live with it. but it is more of a sawed off shotgun BANG!!!
here are the stats again
Dual 42 DCNF Idle Jets 125 Main 40
4-1 header no Cat (or dog for that matter)