what's pilot jet and what's main jet??
The jets in a carburetor are simply holes of a certain diameter that let fuel flow at different rates from the float bowl to the mixture chamber (where the fuel is vaporized as it mixes with air).
The different jets are responsible for the bulk of the fuel flow to the engine AT DIFFERENT THROTTLE POSITIONS. The pilot jet contributes most of the fuel at the lower throttle settings, and the main jet takes over at a more wide-open throttle position. The "needle" dominates in the mid-throttle ranges, I think.
For general info on the topic, see the Motorcycle Carburetor Theory 101 (http://www.motocross.com/motoprof/moto/mcycle/carb101/carb101.html) page, especially the bolded sentences.
For info more specific to the GS500E, wait for Srinath.... :roll:
hah... thanks!