Hi, and welcome to the site. 🙂
The idle speed is not adjustable and is controlled by the ECU when the engine is at operating temperature, with the CTS controlling the fuelling initially. An idle speed of between 750-800 rpm is just fine (this will increase slightly when air conditioning is running).
Worth having a look at the throttle valve (remove the air intake pipe from the air cleaner and throttle housing) to see if it's dirty and clean with carb cleaner if so.
Which spark plugs were fitted? - GM tri-electrode are recommended for non modified engines - part # 9199038
A K&N replacement air filter (33-2771) will help as well.
Hope this may help. 🙂