Okay, I think there's some confusion going on here. The carburetor determines how much fuel and air passes into your engine. A moped needs a specific ratio of fuel to air -- to much fuel in the ratio is "rich" and too much air in the ratio is "lean." Both of these conditions will hurt the performance of your moped, and running lean can damage or destroy your engine. When you put a kit on, more air will be going through your engine. To compensate for that extra air, you need more fuel. The jet is the part in the carburetor that controls the fuel flow. Different sized jets have different sized holes -- the bigger the number/size, the bigger the hole, and the more fuel that will flow into the engine. This is why, at the very least, you need to install a new jet in your carburetor when you put on a kit -- otherwise you will be running lean, and you will probably damage your engine.
When installing a kit, it is better to install a larger carburetor (not just jet) to get the most out of the kit. The kit will draw more air through the engine, but it won't be able to pull all the air it could potentially be using through a small stock carb (think of trying to breath through a really small straw). If you get the bigger carb, you give the engine more air. This larger carb will have the correct jet in it to maintain the right fuel to air ratio, but you will be getting more of that ratio through the engine, which will give you more power.