i just finished mynew engine for my 84z and im having problems with the tach. at first it didnt work but i bypassed some kind of filter or modual on the firewall and now it works but is off about 2000 rpm. i checked this with a aftermarket tach but i would like to keep the original if i can.

i never had the car running before now so i dont know if my tach is messed up or if there is something i need to do to make it read right with the msd box.

 

You've run into a common problem with the factory tachs.There's either a capacitor or a resistor on the circuit board on the back of the tach that is the cause of the problem.My '83 had this problem when I got her. I replaced the tach and problem solved.A decent aftermarket tach will be more accurate but you want to keep her looking original.There have been many posts on this problem and there may even be a tech article on it.
If you have someone you can borrow thier tach to verify this is your problem.And the part you removed on the firewall is a filter for the tach.

 

thanks for the help