Fuel dye
BTW, it is good practice to have a drain hose/line connected to the valve so that excess fuel that drains out goes overboard instead of laying in the bottom of your cowl.
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Opinions, information and comments are my own unless stated otherwise. They do not necessarily represent the direction/opinions of my employer.
Scott McDaniels
Van's Aircraft Engineering Prototype Shop Manager
Hubbard, Oregon
RV-6A (aka "Junkyard Special ")
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