IgnitionAdvancer

An ignition advancer causes ignition to begin earlier in the engine's cycle, resulting in better low-RPM torque. All available mods move the timing up to 17 degrees on the rotor. Carbs do not need rejetting or adjustment, making this an excellent low-cost modification.

Pre-'01 models have a 12 degree rotor for non-CA bikes and a 5 degree rotor for CA bikes. For 2001 and 2002, all bikes were sold with a 5 degree rotor. For the GS500F models, the ignition rotor is different. However, a GSTwins forum member used the DIY modification with good results.

It is located under the right side engine cover.

Vance and Hines

V&H Advancer
Almost impossible to find now, since production ceased a long time ago. Consider one of the following two techniques

DIY

Do-It-Yourself Timing Advance
Only applies to 12 degree rotors (pre-01 non-CA engines).

Bob Broussard

Generous GSTwins forum member who will perform the mod on both CA and non-CA bikes for a small fee. For more info, contact him through the forum.
See this post for more info.