a back firing is a known problem with straight pipes. Its usually cured by a combination of things. An adjustment of the PMS about a half a turn on each carb. If this is the first time they have been adjusted 1) they weren't cleaned very effectively and 2) you'll have to remove the brass plugs covering them from factory. On the left side carb it is on the under side aft of float bowl and right side same place but fwd side of bowl. Removal of carb is necessary. I'll send you links for how to in morning. The other thing you can do is to remove and plug AIS. If you have additional questions, ask away.
2005 1100 Custom
ORKed 5/16" Lines
PODs K&N RU-600s W/PCV
PCS XXX/2 cams
Jetted & PMS Tweaked 20.0/152.5 3.0 turns
Wimmer Machine Custom Oil Cooler & Stash Tube
Dyna 3000 Step 5/ 6500 redline
6" Fwd Control Extension W/Boards F&R
Memphis Shades Bullet Fairing
Progressive 465 rear
Progressive Springs and GVE front
10,000 Monkeys throwing crap at the glass and this one thinks he's Picasso...