Hi there everyone. Im new here. I recently bought my son a chinese import 50cc 4 stroke quad that had not been run for a year. I took it out and it ran perfect untill it got low on fuel when it cut out and would not start. I guessed it had crap in the tank which was pulled into the carb so i dismantled it and cleaned it. From there the quad would make knocking sounds and sputter but idle fine. I was advised to buy a new carb which i have but the same problem remains. The quad sometimes cuts out when i sit on it or when i raise the rear end when ticking over. There are also signs of it running rich on the plug. I have turned the air screw to nearly every possible position with no joy. The quad sometimes backfires aswell and fumes come back out of the carb.

Could anyone please help me out!!!
Thanks in advance