SETTING BY THE LEAD OPENING.
Put the reverse-lever in the full forward notch, and place the engine on the forward center. If the lead opening in full gear is to be 1/16 inch, advance the forward eccentric till the point a ([Fig. 17]) on the valve-stem is that distance away from the tram point. Throw the reverse-lever into the full backward notch, turn the wheels forward enough to take up the lost motion, then turn them back to the forward center. Move the backward eccentric (if it needs moving) till the tram, extended on the valve-stem, strikes the same point that it reached for the forward motion. It will be noted here, that the valve occupies the same position for fore and back gear when the engine is on the center. Put the reverse-lever in the forward notch again, and turn the wheels ahead till the back center point is reached. Now tram the valve-stem again, and, if the lead opening be the same for both gears as it was on the forward center, that part of the setting is right. It is a good plan to go over the points a second time to prove their correctness. But it is not likely that the lead opening at the back end will be right on the first trial. Instead of having the correct lead, the valve will probably lap over the port, being what workmen call “blind,” or it will have too much lead. Let us assume that our valve is 1/16 inch blind. This indicates that the eccentric-rod is too long. We shorten the rod till the valve is at the opening point, and, on turning the engine to the forward center again, we will find that the valve there has lost its lead. But our change has adjusted the valve movement, so that on each center the valve is just beginning to open the steam-port. Advancing the eccentric to give one end 1/16 inch lead will now have the same effect upon the other end; and, assuming that the back motion has been subjected to similar treatment with a like result, the lead opening on that side is right. This process must now be repeated with the other side of the engine.