rpm issue? electronic?

I have gone through the wiring since some ends have been changed and ensured that both cars are wired exactly the same I ran some test with a meter yesterday and the car that doesn't work properly does have a lower amperage and voltage I did a complete tuneup by ear and have it running better now (car with problems has been in a major accident since it last ran well large enough to brake the front frame, wreck fenders inner and outer etc. engine did well with it just hitting the firewall and the blades hitting the rad) I will try hooking up the tach differently and see what readings I get. Thanks for the input.