signal to the carb & the more idle fuel the engine needs. In race applications we like to lock the timing to full advance all the time. After getting the engine warmed up adjust.... After setting you can adjust idle speed up to desired level.Make sure engine is at operating temperature before adjusting. **Idle mixture screws should end up out from 1 to 1-3/4...!!! Make sure you enough initial ignition timing & that your distributor is advancing properly. Very critical. If you have an MSD distributor make sure you use the black bushing & 1...