i have a 86 z28. i did a 305 to 350 swap. the new engine is a 350 l98 aluminum heads/manifold. my problem is very often my idle will get hung up at 3000 rpm. i have to let it idle there for a min or so then floor it and then it will idle normally. what is the problem. i have a rebuilt rochester quadrajet. jetted for a 305. only 2000 miles on the carb. im stuck please help.
5 speed trany. electric choke, computer controlled carb.

 

This is actually normal belive it or not. The carb sences that the engline is cold and so it will actuate a few different mechanisims. One is the choke plate, which reduces the airflow into the carb, ritchening the A/F ratio. Another is the throttle solonoid (It could be vaccum or electric) which pushes the throttle open to help accelerate the warming of the oil. Another thing that will happen when the cold start mechanisims kick in is a rise in engine idle. Usually, it is factroy set at between 1200 and 1500 RPM. But since you have a rebuilt carb, chances are, you just need to reset the cold start idle, which is achived at the throttle solonoid or choke plate. There are so many different models of these devices, it is hard to give instructions without seeing the type of solonoid you have, but for the most part, find it and disconnect it, and that should end your troubles

 

You didn't say whether the idle was high when starting up with a cold engine or whether it would be at that high idle speed when you pushed in the clutch while stopping. My 81Z28 (350/4spd) did that once. The throttle shaft for the front two barrels was bending/breaking inside the left barrel. I was really lucky there was enough thread to hold that screw in place. Just a possibility if its hanging up at high idle and you notice it as you push the clutch in.

 

its not when the car is at a cold start. i can start the car and it will fast idel to get warm. then ill kick it down, by pushing on the gas. it will idel fine then. the problem is when im driving, and all of a sudden, when i let off the gas th rpms hang and wont come down. i have to wait a few then slam the gas and then it will idel fine. i would like to slove this problem andy suggestins are welcome.