300ZX Project car getting closer

1985 2x2 project car is about to hit the road. My son has been babying it for a few months. We are trying to get the registration straightened out and the alternator was bad, but it runs pretty good now. Boy, if the alternator is not producing volts, the dash doesn't work right. Anyway, we got a used alternator that looks like it was just rebuilt for $40. A little tricky to replace, but not real difficult. Turns out we have another problem. We replaced the possitive battery terminal and cleaned the ground connection from the battery. Everything worked much better, but after a couple of days, my son says the engine won't rev above 2100 rpms. Haven't had time to take a look, but does anyone have any ideas?

Go through the ignition system...I'd start there.