It has a CV carb so it could be a diaphragm problem. Havent done much with GN but motor is very similar to SP400.
First thing is to get the bill from where you had carb done and find out exactly what they did, then tell them you want it done right.
You also want to check all the ground leads at CDI, frame and everywhere else. If its running it isn't the CDI (in my experience)
PJ