Every cheap Chinese quad I have ever seen has had problems of some sort, every single one. Once the problems start, they usually don't stop. In my experience it has been one thing after another in every case. A friend of mine bought one for his nephew, it has had carb and chain adjuster problems since he got it. His girlfriend bought two more for her kids, both of them have been problematic from the get go. One of them broke the primary driven gear the first day they had it. I hate to be the one to sit here and say it, but it is best for anyone who has one of these Chinese machines to try and sell it before it gets so bad that nobody will possibley buy it. I am not trying to brand bash or put down anyones choice of machine, I am just giving what I think is my best advise. I would rather pay twice as much for a good used Japanese or American machine than spend my money on any of the many knock-off Chinese brands. That being said, I agree with the stuck throttle, it could be the carb slide hanging up, a pinched or stuck cable, or a problem in the throttle housing. Good luck.
Yours truly, sincerelly, love always, Josh aka The DirtDemon.