I copied this off of the internet somewhere, maybe it will help.
Note: Before adjusting carburetor, be sure engine is at operating temperature, (choke fully off) air cleaner is removed and vacuum lines are plugged off.
1. The Weber DGV/DGAV idle speed screw should be adjusted to its preliminary set point before adjusting the idle mixture. To set the idle speed screw, follow these steps.
2. Back "out" the idle speed screw until the tip of the screw no longer touches the throttle lever. Then slowly turn the screw in until it just comes in contact with the throttle lever.
3. From the "contact" position, turn the idle screw "in" one (1) full turn.
4. If a tachometer is available, install it prior to starting the engine. If a tachometer is not available set idle mixture "by ear."
5. Start engine, be sure choke is not engaged, and proceed to adjust the idle mixture.
6. Turn the idle mixture screw "in" (clockwise) until the engine RPM begins to fluctuate on the tachometer. (If adjusting by ear, until a noticeable drop in speed is heard.)
7. Back "out" (counterclockwise) the idle mixture screw slowly, until the engine idle becomes steady. Try to obtain the leanest setting without affecting the idle speed. If necessary, repeat steps 6 and 7 until the best setting is achieved.
8. Once the mixture is set, fine-tune the engine's idle speed, if necessary, by readjusting the idle speed screw. (Note: Turning "in" (clockwise) the idle speed screw will increase engine speed. Turning "out" (counterclockwise) the idle speed screw will decrease the engine speed.)
9. If idle speed is reset go back and repeat steps 6 and 7.