1993 F250 idles too high

I Would say the choke on your one barrel carburetor is sticking

Start the engine. Let the engine reach operating temperature.Remove your Air cleaner to your Carburetor while the engine is running.Look down the throat of the carburetor and you will see a round flap A.K.A the butterfly.This flap should be completely open when the engine is at operating temperature open so that the air can pass through the throat of the carburetor freely. If you don't understand how a choke works go to howstuffworks and ask how a carburetor choke works and it will all make since.