If the ATV is stock, then stock size jets should be fine. Just make sure the jets in your carb ARE the stock size. Someone could have changed them before you got the ATV. You may still have some debris in your carb. Sometimes it's easy to miss something. The fact that it had been sitting for a year or two makes me think the carb is the problem, but I could be wrong.
Most 4-wheeler problems are caused by a loose nut connecting the handlebars and the seat!!
You only need two tools in life -- WD-40 and duct tape. If it doesn't move and should, use the WD-40. If it shouldn't move and does, use the duct tape.