Sr125New

Hello everyone!!! I'm a new member of the forum and also I'm and a new driver. Never ride a bike and I just bought Yamaha sr125 custom. Any way I like it very much it's easy for driving very comfortable but I've got a problem. So the problem is that when I'm driving and approaching the traffic lights I'm start shifting down to slow down and stop then the motor cut off. So the question is am I doing something wrong or my machine got a problem?