When i cold start my 84 z28, it revvs consistantly at 1500RPM until it is warm enough and i tap on the throttle. 9 out of 10 times when i start my car and it is running at 1500RPM i hear a consistant tapping noise that gets faster when slightly above 1500 and slower when slightly below 1500. What could this noise be, it sound like it is coming from the passanger side? The tapping goes away after the car is warmed up, or when it is anywhere else but 1500RPM. I originally thought it was a warming up problem, but sometimes *rarely* but sometimes when it is at opperating temperature, 220 degrees, i will hear the tapping at the same 1500RPM. I dont know if it is an exhaust leak, because i did swap out the manifolds for headers, or if it is a valve tap, or somethin else. All i know is it didnt do this in the summer when i first started drivin the car. When i put the heads back on i tightened the valves a quarter turn past BTDC and i have driven the car 3000 miles since and it has had 4 oil changes already, i no thats alot of changes but the first 3 were within the first 1000miles because i was puttin in some of those engine de-gunkers and i wanted to make sure there wasnt any more of that crap in my oil. I used Mobile1 10w30 everytime.
Please help, im puttin the car away for the winter on monday, but when spring rolls around i want to have a plan of attack to fix this problem.

 

The fast idle makes sense. That's the fast idle cam on the q-jet. It's supposed to do that.

As for the tapping, sounds like a valve lash setting problem. Reset the valve lash 1/4 to a 1/2 turn past zero lash, not in relation to piston location.

 

Might be an exhaust leak. Check your header bolts and gaskets. Headers have a nasty habit of coming loose.