A You are correct in that a conventional two-cylinder steam locomotive exhausts (“chuffs”) four times per wheel revolution. All exhausts should be equally spaced regardless of the position of the reverser, speed, and how hard the locomotive is working. You are also correct in hearing that this is not always the case. When the locomotive is new, or freshly shopped, the valves in the cylinders will have been “set” or timed by a machinist skilled at “valve setting.” If the valve-setter does a good job, the exhausts will sound “square” (90 degrees apart) no matter where the engineer places the reverser during operation.
However, it is inevitable that the various parts of the locomotive will wear over time, and they do not wear evenly. As worn conditions develop, the locomotive will no longer sound square, with one (or more) exhausts not as “crisp” as others and/or sounding “mushy.” Occasionally, this will be more pronounced in one direction than the other. Unless corrected, eventually, the locomotive will be “out of square” or “lame” and will have a difficult time starting and hauling a heavy train.
Due to the number of variables which must be taken into consideration and the various parts which must be machined or modified to fit the particular locomotive being worked on at the time, proper valve setting can be considered almost an art. – J. David Conrad, vice president and chief mechanical officer, The Valley Railroad Co.
