Well, first you need to read up- do a search on this forum and look for information on jets.
The jet on a carb is the passageway for the gas. It determines the rate of fuel flow and atomization of the fuel. Whenever you change something on a 2 cycle motor, you change the amount of fuel the motor wants and needs. If the engine needs more fuel, you put in a slightly bigger jet- less fuel a smaller jet.
You determine this by doing plug chops, which are described very well in Fred's guide, under resources.