What should you do when starting an idle air compressor?

When starting an idle air compressor, manually throttling the compressor suction valve is crucial because it helps control the flow of air entering the compressor. This action can help prevent surging or instability during startup, ensuring a smooth and safe operation. Proper throttling allows for a gradual increase in pressure, which ensures that the compressor can adjust to the working conditions without overwhelming itself or putting unnecessary strain on the system.

In the context of the other options, opening the compressor discharge valve without properly controlling the suction could lead to sudden changes in pressure, which may not be safe. Adjusting the pressure settings should be done after establishing a stable operating condition rather than at startup. Turning on the refrigeration unit may be part of the overall operation but does not focus specifically on the correct procedure for starting the compressor itself.