got this slow speedfight 100cc that needs help?

hi, looking 4 help on a speedfight 100, accelerates smooth up to 40 then from 40 to 52ish it accelerates more quickly but there it tops out unless there is a downhill bit involved. The only mod it has is a Leo Vince Touring exhaust. I bought with this exhaust on so I have never ridden it any different. should it accelerate like this or is it faulty, can anyone help - thanks - Rod