I did not realize you hadn't made your basic adjustments. Here is a video to help you understand which screw does what https://www.youtube.com/watch?v=yX_hCujFQ_I and one from Harold's HO page to help you understand. https://www.firebird400ho.com/carburetor-adjustments/ The other thing I was talking about- the cable being to short and not allowing you to get base idle is something to check besides a vacuum leak if you can't get your base idle. remember to only make the base ide adjustment after the engine is up to full operating temp. As far as fast and curb idle speeds use what Pontiac specified most people with stockish motors are happy with that. Here is the service manual to help out https://www.firebird400ho.com/mdocuments-library/ if after getting all your base adjustments correct and making sure your throttle cable is not holding it open you still have your rpm problem it will be easier to narrow it down.