The Gauges simply illustrate the rate of fuel flow in USG per Hour. The flow rate is controlled by the Mixture Controls and appropriate flow rates for different phases of flight are shown at the back of the Checklist.

It should be possible to discern a small movement of the needles when priming the engines prior to starting, but it is sometimes not easy to see. Typical flow rate at idle will be about 5 USG per hour. (per engine).

If in doubt ensure that the flow rate is more than sufficient to assist engine cooling, and monitor CHTs at all times with this in mind.