What causes a 1987 Nissan Sentra to rev high when starting

The cold start function, commonly known as auto choke, is a feature that would increase the engine rpm during starting and if engine is cold, the rpm would remain high for a short while before coming down.

When motor is warmed up, it would still go up before settling down as the engine computer needs a second or two to understand the engine temperature condition and recalibrates iotself. If the idling speeds remains high, it could be a fault with the IAC or engine coolant temperature sensor. Insufficient coolant in system would cause that as well.

Yes, carburetted engines do have auto chokes. Ensure the wire for the auto choke is functioning and the choke opens fully when engine is warmed up.