What would cause my Harley Sportster to back fire and shut down?

I was driving down the street got up into third gear when the bike back fired and puttered out. One restart it just wouldn't fire up. I changed the spark plugs and charged the battery, I can smell gas so I believe it's getting gas.

In a vast majority of cases where back fire occurs, the culprit is usually the spark plug.However you mention that you have already changed it. Still I suggest you get yourself a brand new set of plugs of the correct matching heat range. Then gap them correctly before you install them. Also check your ignition switches. You may be accidentally switching off when riding. A richer mixture will help you start easily.