The modern Vespa scooter model, the GTS 250, comes stock with two rear shock absorbers that can be adjusted by the owner as desired. The front shock cannot be adjusted, however. The first rear shock position is primarily for a single rider's weight. Positions 2 and 3 provide better support for a heavier rider, and position 4 assumes two riders on the scooter. Given the design, a Vespa owner can adjust the dampening for multiple weight conditions as needed rather than having to settle for a non-adjustable ride.
Place the scooter on its kickstand in a clear area where you can bend down underneath the scooter. Position yourself under the rear of the scooter near the wheel so you can clearly see the two rear shock dampeners and their bottom halves.
Check the current setting of both rear shocks to determine whether to loosen or tighten them as desired. Use a shop rag to clean the shock adjustment ring area so there's no grease or debris before you try to make any adjustments.
Use the dealer provided shock adjuster wrench and hook it into the adjustment ring to adjust the setting on the shocks. Start with one shock and pull or push the wrench as desired.
Turn the wrench counterclockwise to tighten the shocks (heavy-rider or two riders) and clockwise to loosen the shock spring (single-rider). Do not twist the shock adjustment ring any further than the design allows. Make sure the corresponding second shock on the other side of the wheel is adjusted to the same setting as the first one.
If you don't have a shock adjustment speciality tool, you can either buy one from a dealer for a few dollars so that you're using the right tool, or you can make the change with an adjustable claw wrench. However, keep in mind the claw wrench may slip while making the adjustment and could cause tearing on the adjustment ring, making it harder and harder to adjust over time.
Do not try to over-tighten or over-loosen beyond the adjustment range of the shock ring. You could possibly slip and gash your hand from the sudden loss of leverage.