DETECTING THE CAUSE OF A LAME EXHAUST.
If in moving the engine ahead slowly, with the cylinder-cocks open, it is found that steam is admitted to the cylinder before the piston has nearly reached the center or dead point, or that the back cylinder-cock does not show steam till after the piston has passed the back center, the eccentric-rod is too long. The rod being too short produces precisely an opposite effect. The steam arrives late on the back stroke, and ahead of time on the forward stroke. This is different from the action of the steam where an eccentric has slipped. In that case, there will be pre-admission of steam before the beginning of both strokes, or post-admission, that is, late arrival of steam, for both strokes. Take a go-ahead eccentric for example. If it slips backward on the shaft, its effect will be to delay the admission of steam till after the beginning of each stroke; and, if it slips forward, the result will be to accelerate the lead of the valve opening the steam-port before the piston has reached the commencement of each stroke.