Ever since the motor was swaped into my 86 IROC Z the tac stays at about 4-5000 rpm.
When you drive it pegs out,did someone hook up the wires wrong?? somewhere in the eng compartment???

 

Depending on the old and new engines, you could have a counting problem, or voltage problem. If the old engine was a four or six cylender engine, and the new one is an eight cyl., you will need either a chip or a reducer added on to your old tach or computer. If the old engine had fewer cyls., the computer is still counting that number, so every time 6 or 4 cyls. fire, it counts 1 rev as opposed to 1 every eight. Another problem might be the voltage. If the engine you swapped in was the same number of cyls. as the old one, the distributor might be putting out a stronger signal. Try inserting a reducer in-line to the tach to rectify this.

Hope this helps,

-Berlinetta00