Tap-tap-tap it on a hard rubber surface (such as a tire)
Fred,
Hold the motometer in your hand like a pencil (the brass part, that goes in the radiator top, should be down). Rest/position your hand on a hard rubber surface, such as a tire, as if you were going to write with the brass end of the motometer. Then rapidly tap the end of the motometer (at the end of the part that goes in the radiator top) against the hard rubber surface while holding the motometer at approximately a 45 (or more) degree angle (from horizontal).
That tap-tap-tapping will cause the red fluid inside the thermometer tube to eventually drain down to the bottom of the thermometer. I have found if you hold the motometer at a vertical position while tapping it on a tire, the red fluid will eventually descend as well, but will stick to the sides of the thermometer tube and thus not flow down as fast.
My father has a NOS motometer with it's original instructions that indicates that the red fluid may separate in the tube while the motometer is in shipment. The tapping process that I describe above is what the original instructions say to do to fix the red fluid separation and to force the fluid back down the thermometer tube.
I have done this process many times and it works well.
Fordially,
Brad in Germany