Newbie saying hi and needs help

[chink]

Hi everyone. Great site you have here. I'm new to scootering and I have a kinroad XT50QT. Derestricted it by cutting the pink wire and raising the needle. There weren't any of the other common restricters. It would do 38 mph after the derestriction but now will struggle to hit 28mph. I was getting a slight backfire when coming off the accelerator but this has now stopped. All I have done since the derestriction is change the oil to semi synthetic. I have adjusted the valve clearances, checked diaphragm, air lines, fuel lines for hair line cracks and splits but everything is perfect. I then attempted to adjust the mixture and idle but this is when things got worse. I let the scoot warm up for over half an hour and then turned the mixture screw completely inwards. The engine continued to idle as normal without dying!! I thought maybe the cold start was still on so I rode it around the bloke a couple of times. I then attempted the adjustment again but all I got was the same problem. I removed the cover off the cold start and it was so hot I couldn't even touch it. Now the scoot starts when cold without the cold start (I've disconnected it). Tried adjusting the mixture again with the disconnected cold start but I can still turn it completely inwards and it idles fine. It bogs down when accelerating and takes all day to hit 28mph.I've cleaned the carb out twice also.
Any help to fix this is greatly welcome,

Dan