What causes 1990 Mazda miata high Hc on idle?

In very laymen's terms, high HC numbers mean that your fuel isn't burning off as much as it should and remainder is making it out of the tail. This is either because you're running rich (the car's ECU thinks it needs to inject more fuel in the engine), or more commonly, your car has weak spark so the fuel isn't being burned.

Miatas typically run a bit rich, but tend to respond very well to a new set of plug wires and spark plugs. The run-of-the-mill NGK blue spark plug wires and basic NGK spark plugs at $2-$3 each will work just fine for your tune-up.

The more expensive possibilities are that the sensors are now 20 years old and not giving the ECU proper information. Sensor failures on these cars commonly lead to the ECU thinking it needs to inject more fuel. Culprits may be the Air Flow Meter/Air Temp Sensor, O2 sensor, or bad EGR or IAC valves (the valves can get crudded up and seize, making the engine unable to manage incoming air). The valves can typically be cleaned with sensor-safe throttle body cleaner.