MIL cause rich condition?

Just wondering, now that I have a new exaust and new o/r h-pipe when shifting a high rpm or WOT fron 2nd to 3rd i get a muffled pop through the exaust (sound like driver side but cant stick my head out the other window ) and was told it was because it is rich. Will the MIL cause this since I have not recieved my MIL eliminators yet, hope they come soon I ordered them a week ago
BTW it is a Flowmaster American Thunder cat back with BBK O/R/ H-pipe and stock manifolds.