I have a 1989 Honda Accord with a carb. It starts ok but idles rough, around 800-1200 rpm. When it warms up it idles in gear at 1200 but in park around 23-2500 rpm. When the ignition is turned off, motor diesel’s and does not want to stop. The issues have slowly gotten worse (Progressively). I’m a novice with a Haynes manual for repair.
I think you should start by looking for a vacuum leak. Many times you can hear leaks by just poppong the hood with the engine running. Grasp vacuum hoses and give them a shake to see if the engine’s speed changes. If it changes, track the leak down on that particular hose. Sometimes I use a 2’ piece of garden or heater hose held near my ear as a stethoscope. Move the other end near possible vacuum leak sources.