Have you ever checked your valve clearances? They might have tightened up. Common in the 660 engine. This is the most likely culprit.
Do you ever store the ATV or have it go long periods between uses? If so, do you drain the carb of gasoline or use fuel stabilizer? Do you regular gas or ethanol mixed gasoline?
Today's gasolines break down fairly quickly, especially ethanol fuels. You carb might be dirty in the pilot circuit, and this can give backfiring on deceleration.