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I have a 2013 Jonway YY250TT scooter with the CF250 engine. The scooter is popping through the carb when it is running at 4000 rpms to 5000 rpms. It also won't hold steady at thoughs rpms and will actually surge with bigger than a 138 main jet is installed. I have been trying different jet combinations to see if I can clear it up but it will only get worse but not go away with the different jets. I don't find any air leaks at the intake or carb area but I do have a small leak at the exhaust flange I can't stop. I even tried a different cab but it still does it. I also noticed that I have very little response at my air fuel mix screw. I can turn it all the way in and it will still idle great no matter what size jet I put into it. It has always done this with the air mix screw ever since I found a tool that could get to it and that was about a 1000 miles ago or so. I've have 112 to 144 mains in it to test different combinations. Do I have a valve problems or what is going on? I have 3300 miles on it. All highway miles.