Thanks for all the suggestions, everyone! I started with the spark plug, and it did look a bit dirty, so I cleaned it and made sure the gap was correct, but it still didn’t start. Then, I checked the fuel lines and the gas tank vent hose, and they seemed clear. Finally, I had a mechanic do a compression test, and it turned out that the compression was too low. He adjusted the valve clearance and the ATV started right up! It seems the valve clearance was out of adjustment. I appreciate all the help; your advice really pointed me in the right direction. Thanks again!