Linhai 260 ATV model: XT260 While motor is running can't shift into high-low or reverse. Makes a terrible grinding noise and won't go into gear. With motor off it will shift to all gears and travel in that gear. What will fix that?

 

Not familiar with the model but seems like a stuck clutch. If it has a handlebar mounted manual clutch, broken cable or the plates stuck, or just needs adjusting. If it has an "automatic" clutch, the idle speed has to be slow enough for it to disengage, if idle is just above stall speed and it still sticks, it will be the centrifugal clutch or the "one way" clutch inside it, which has stuck, and you will need to take the cover off to inspect it. Some belt drive autos don't have any clutch, the belt serves as a clutch, a centrifugal set up makes the pulleys move apart so the belt goes slack and slips at idle. If it is this type, the pulleys must be sticking.