I have a 1984 atc200es and it runs but idk what to put the fuel petcock setting on, on,off, or reserve?

 

On position,reserve is in case you run out of fuel. Reserve position can at least get you back to where you're going as it draws fuel at the lower part of the tank. You leave it on reserve and run out of gas,you walk. Best to turn all these old machines to the off position when you're through riding. Less chance of fuel bypassing the carb into the engine if the carb needle and seat are worn.