A Wilbar tube is a glass tube with very small inside diameter and marked with graduated lines on the outside. The tube is mounted in a plastic housing with a threaded fitting on the bottom that is the same thread as the fitting on the high pressure fuel
lines. To use the tube, you remove the high pressure fuel
line for the appropriate cylinder from its connection at the injection pump
and thread on the Wilbar tube. Then you roll the engine
over a few times until you see fuel moving up in the center of the tube. As you rotate the engine in its normal direction of rotation the plunger in the injection pump will rise and when the plunger top closes the internal fill port, (port closure), pressure will immediately build in the pump and the fuel in the Wilbar tube will start to move upward. This corresponds to the start of injection and is the same point at which the timing marks on the flywheel and timing tool should line up.
Sometimes a glass Wilbar tube isn't used but a steel
tube is used instead. The steel
tube looks like a short straight piece of high pressure injection tube with a steel washer braised on its top, and the washer has a very small hole drilled in it. The timing procedure is the same as the Wilbar tube, but instead of the fuel moving up the glass tube, the fuel begins to exit the hole and flow onto the washer, and the beginning of the flow is easy to discern.
Depending on whether the timing is early or late you add or remove shims/gaskets from below the flange of the injection pump to alter the timing.