Yup dirty carb.
could be as simple as a little water in carb bowl.
Try draining carb bowl by the screw at very bottom of the bowl.
With the fuel tap Open from gas tank , lossen screw.
This should flush out old gas, dirt and water.
You should do this on a semi regularly basis.
If this doesn't help , then you will have to pull carb and clean manuely.
Used ATV's are like Hookers.
You can find a nice one, bring her home
and marry her, but you will never know the hours
or the beatings they have taken.
Disclaimer: i have never married either.