Ok I got a 75 280z it idles really rough at about 1100 rpms (cant get it to go lower) I just replaced all the sparkplugs (ngk) and the air filter. The spark plugs and cap are farly new too. What is going on here.

Also some times I start it up and it just chokes, backfires and dies then other times it just idles rough:sick:

Sounds like it may have a problem in the cold start valve (injector) or one of the sensor that controls it. You really need to get a Factory Service Manual for your 75. It will give you step by step instructions on how to diagnose and test the various systems in the fuel injection. It is expensive, but worth every penny when diagnosing FI problems.

I found that vacumm leaks can cause these symptoms. Such as a loose oil fill cap, and/or a break or hole in the large (about 3/4" diameter) rubber hose that's below the intake manifold.

(having/getting a manual is a good idea!)