The '78 280Z is running well. But the strange thing is that as the RPM climbs, the tach tops out around 3900-4000 RPM - the tach, not the engine. What's up?

The '78 280Z is running well. But the strange thing is that as the RPM climbs, the tach tops out around 3900-4000 RPM - the tach, not the engine. What's up?

Stock setup or updated ignition or distributor/pickup?

Either a tacho defect in the driver circuit or the tacho input signal. What happens after 4000 RPM, the tacho needle just stops there? Does the engine feels normal at higher RPM, or does it start to run rough? The easiest way for a diagnosis would be to try another, known to work tacho. Ideally you check out the tach trigger signal with a scope to check if the duty cycle of the square wave or the amplitude changes after 4000 RPM.