how to get the rpm and timing right without accelerating the engine too much??

Im setting idle speed and making sure timing is correct.Thats what the underhood label says: idle speed 750rpm and timing on 6 degrees. Just dont understand what is that ground wire connector for ? why do i need to ground it ??

That is for setting the 'base' ignition timing only. Disconnect , then adjust idle. What else was done , leading to the need to make adjustments?