Plugged carb or plugged pipe.
That's my first thoughts.
When on a block, does it rev up and seem to run fine?
Put your hand over the exhaust and see if it's blowing out good.
Either way I'd clean the carb.
2000 GNC 5N\
2001 GNC 9N \ 40+ class
2004 GNC 9N /
2014 rock crusher "GO tortoise GO!"
2016 Byrd Rooster
Father of the "11" time National champion,and worlds fastest woman ATV racer
A few fast LT80's
250R~300cc's
300ex
350 Rancher
Kymco Mongoose
Survivor of the 12 Hours of ATV America 04 and 05