Help with a Grizzly 350

I'm trying to do a little research for my buddy. He's got a 2009 Yamaha Grizzly 350, and he says its not revving up and has no acceleration. He said when he starts it and lets it idle it runs just fine, but if he tries to give it any throttle, it acts like it hits the rev limiter. He's done some research himself and found where some people had replaced the CDI box. With the research I've done, I read where possibly brake cables or shifter cables were out of adjustment. I'm going to take a look at it for him and see if I can figure something out. Obviously I'm going to look at the carburetor, plug, etc. first to make sure it's not something simple. I'm wondering if anyone else has had this problem, or possibly can point me in the direction of what could be wrong. Thanks for your input and help

Help with a Grizzly 350