Mini Quad S4 Cobra

[Ray6105]

Hello there pretty new to this chat stuff. Alright to get right to it. I just came across a mini quad S4 Cobra maybe a 2000 or so version, not a 2017. Anyways someone was throwing it away, but I salvaged it and got it working. My latest problem is the exhaust is smelling of rotten eggs and burning at the very bottom. Any suggestions as to what to do, or how to fix it would be greatly appreciated. Thank you