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.

Thank You for your response what would be the cause if were to happen after moter is warmed?

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.

Thank you again for your response your help is greatly appreciated does these solutions apply to both fuel injection and one with carburetor? I have a carburetor on my nissan.

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.