That's the air metering plate...
On older/high mileage/loaded up engines I use water to clean them out w/ Good results.
Get car up to full operating temp, I use a water bottle w/ a small hole poked in the cap and work the stream/RPM's so the car just has a Touch of roughness while running. Do not cut rpm's till the engine smooths/works through the water that's in the system.
Always change the oil afterwards, it will be Black as Night!!
Jonathan