I have a 89 Iroc with a 305 in it and a 5spd and the idle is irregular. Is that normal or could the previous owner have put an after market cam in it?

 

Could be. If the idle is like a wumpity-wumpity, then that could be a cam. If the idle speeds up the rpm's, then slows down and keeps doing that, then it's something else causing that.

 

The rpms jump around from around 1100 to 1500. Also, when I slow down, the car shakes unless I keep it in neutral.

 

Has it ever had a tune-up? If not, replace the plugs, wires, cap & rotor. No guarantees, but your engine may just thank you for that. My old 91 (305 TBI) had an erratic idle when I first got it. So I replaced all those ignition parts. I was curious which was causing the bad idle, so I tested the engine after replacing each thing. It was the cap & rotor causing the bad idle on mine.

 

thank you for your advice. i changed what you said to change and the car runs good now.