1985 GMC C1500 truck idles too fast

This truck just stared idling too fast. When it starts, it is fine, but once you touch the gas, the idle goes up and won't go back down.

If I take the air filter off, I can manually flip a lever down on the left side of the carbureator and the idle goes back to normal.

When I give it gas, the idle goes back up and stays.

How do I fix?