Form what I understand, Lloyd sells an idle air control valve that will reduce the backfiring on the closed loop bikes.But it doesn't completely
get rid of backfiring. Since he did your work, why not give him a call. He is very helpful.
"05 Kingpin Deluxe,100 inch kit with 11:1 pistons,stage III cams, 2-1 RPW pipe,S&S airbox,PC III,Lloyd's rev extended ECU.