I just bought a Coleman. Like new from a friend. My son has been driving it the past few days. Today, he said it slowed down, started blowing white smoke and stalled. It was hard to pull to get it started to get it home. Every pull, white smoke blew out. It's all stock. Only thing I did was unscrew the throttle screw most of the way. I assumed that gave it more speed. What do you all think the issue is? New to mini bikes as you can see. Any help is appreciated.