Jet Kits offer replacement needles, jets and sometimes springs, in combinations that have been tested to improve throttle response, and help your machine make more power. In the common CV style carburetors on most bikes, needle profile, size, taper all affect fuel delivery in the low end and midrange of your throttle position and rpm band. The main jet controls high rpm and large throttle openings for maximum power, while the pilot jet helps control low rpm and off idle throttle response as it transitions to the needle. Some tuning may be required for best performance.