Why would I want to change my jetting?
From the factory, carbed bikes tend to be set up in the following way to make it through the EPA hoops. See the other parts of the Jetting section for an explanation of terms.
You will need to rejet if you make any changes to your exhaust or intake. If you add a new pipe and/or filters, your carbs will run lean, so you'll need to richen things up. A carb kit will do this by giving you larger main jets and adjustable jet needles. The jet needles also have a slightly different taper than stock, which will smooth out throttle response through the whole rpm range and add quite a bit of power in the 3,000-6,000 rpm range.
If you don't rejet and run your bike lean, it will a) not run without the choke being partially on, and b) may burn (melt) your exhaust valves if you run it hard lean.
If you are going to spend the $400+ to add the pipe and intakes, go ahead and spend the money for the jets and do it right.
One thing to consider
Jetting is really more art than science. Please read ALL of the other articles in this section before you start changing your intake and exhaust systems. It takes quite a bit of trial and error.
Also, please heed this note from one of our resident mechanics: