remember, these machines need a certain balance of gas and air, if there is not enough gas in the mix, they are called lean, if there is too much gas, they are called rich.
the jets are what effect how much gas is put into the engine.
ANY time you change something on the quad that effects how much gas or air it uses, you need to consider whether the jets are still ok, and re-jet if needed.
a new air filter will let more air in, but not gas...
a new exhaust will let both air and gas through faster, and will require more gas for the mix.
etc etc... so the stock bike is lean, if you change anything, it will need to be adjusted.