One of the cars I'm working on wouldn't rev above 4k (not mine, a customer). Well, turns out that hooking up the tach wrong was the culprit. I mixed up two wires (red/back & black/purple) and that was the whole issue.

Now if only figuring out the problem on my car was that easy.

Great for you Mark.
But I need to here more!
My tach stays around 5000.
Does that mean that the po wired it up wrong also?
Thanks John

Now help out Andy...