pitbull150

Hi All, I have recently purchased a beta alp 200cc (2003). the bike is totally standard other than the rear silencer has been cut down professionally and re welded. the problem i am having is that when i throttle off the bike pops back and backires through the exhaust. i am asuming that it is running to rich? i have tried to look for a PDF manual on the net for settings etc but cant find one. can someone please let me know if one is available or the best way to resovle the problem. the bike starts and runs fine other than this issue.