jets are things that are in your bowl of your carb. some are round and some are hex shaped. they ley gas into your engine. bigger jets=more fuel. you take off the bowl to your carb and your main jet is right there and your pilot jet is harder to get to. but you need to change them if you upgrade parts on any motorcycle.
parts like aftermarket pipe/silencer, reeds, (you dont have to rejet for these two, but i roccomend it), big bore kits, after market air filters... i have mods in my sig and i am running 320 main jet. it came with a 300 but i felt that wasnt enough
im getting an fmf fatty gold series pipe. what should i adjust the main jet to? i dont think i would need 320 because im afraid that would be a little rich, but its always better to be rich than lean i guess