There are so many possibilities. OEM plug wires have carbon based cores and eventually break down, carb. diaphragms tear or get stiff and can cause rich conditions, rings and compression can be worn. Tell us as much as you can about the bike, the history that you know, and if the problem happened gradually or quickly. Don't worry about writing a long post, there's good troubleshooters on this site, and detailed information is their ammo.