High and Low Speed Test.
If the control valve is opened too far, the engine will accelerate to a point where the speed becomes erratic. It is like it accelerates and runs out of fuel and slows down... then accelerates and runs out of fuel and slows down... repeatedly. The control valve is then adjusted (closed slightly) to obtain a steady high speed.
A little difficult to hold the laser tachometer steady on such a small flywheel.

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