Why my peugeot Jetforce backs from exhaust when giving gas on engine break ?

Can someone help me out plssss ?? My Peugeot JetForce 125cc make noises from exhaust, like puffing/backs when it's out of gas and going on engine break, as soon giving gas again, it will do the noises. Once you gain power again it's stops and runs nicely. I tried to change the exhaust pipe, but all remained the same. it's a good bike, but this thing is making me sick !!!!!!

Brian
March 2009