Mine used to do the same thing...the problem is the Kawasaki Clean Air System...KLEEN...or AIS (Air injection system...they are just different words for the same thing) Basically it is just the system injecting air into the exhaust side of the valvetrain so that when it exits the motor through the exhaust, the excess/unburned fuel mixes with the air and is burned off by the honey-comb catalyzer in the stock exhaust (since fuel needs air to burn) and since the catalyzer gets so hot, it ignites the fuel/air mix and keeps pollution down. It is just a smog control issue. When you put the slip on onto the bike you removed a vital part of the system...the honey-comb catalyzer in the stock exhaust so when the air/fuel mix comes out it doesn't burn off as soon. It just builds up and when you goose it and that basically causes it shoot out in the cool "flame thrower" you spoke of! It also happens alot under deceleration. It really won't hurt anything but the backfiring does tend to get on your nerves! You can remove the AIS pretty easily...takes about 45 minutes tops. Just do a search in the performance/how to section for KLEEN/AIS Removal...there are pics and instructions for removal.