how do i stop my bike from back firering?

[Jesse B]

i have a yz250f 2002 model and when i back the trottle off it back fires. it has a standard pipe but it has a long skinny dent underneath it, would have something to do with it? i have some other pipes and mufflers but dont want to put them on because they will make it even lounder and faster and i just got on the bike and im only14