The most common problems with the plastic fuel flow sensor not giving a reading (or given a low reading) are:

Restrictor jet not installed correctly: The small restrictor jet is included with the flow sensor and needs to be installed in engines with fuel flow rates below approx. 10 gph (there are 4 jets - more on them in this article). This jet needs to be pressed into the intake port of the flow sensor with the back of a drill bit or similar, until it comes to a clear stop. At this point the nipple of the jet will be protruding slightly into the flow chamber, as a reference. The most common error here is that the jet is not installed all the way, because it can be a little difficult to press in. It requires strong steady force. When it is not pressed in all the way there is not enough fuel pressure to spin the impeller sufficiently and the fuel simple flows around the impeller.

Bubble in the chamber: The flow sensor should be installed facing slightly upwards (45 degrees recommended) so that any bubbles that collect flow through the system. If there is a bubble in the system, run the engine at high RPM until it clears. Note that if the sensor is not protected from vibration, bubbles are more likely to collect. Rubber stand-offs should be used to mount the sensor.

​