Re: A "Barking" rhythm trivia question

It is very rare to find a locomotive that will run with no noticeable exhaust pattern at all (i.e. run “square”). Variations can be caused not only by careless valve setting, but by normal wear and tear in the various pins and bushings of the valve gear and in the rest of the running gear, by leaking rings on the pistons and piston valves, and in some cases by poor design of the valve gear itself. Often, it is a combination of the above factors that give the end result.
Often, proper valve setting requires adjustments in the length of various valve gear parts of 1/128th of an inch. This when the lost motion in the various parts exceeds that amount. Adjusting the wedges on an engine with Stephenson valve gear can throw off the valve timing, without a finger being laid on any part of the valve gear. Overhauling a split brass main rod to a different center to center length on an engine with Walschaert valve gear can throw the valve timing out of square, also without touching a single part of the valve gear. One leaking steam ring on a piston valve will cause one exhaust to be stronger than the other three. One leaking exhaust ring on a piston valve will cause on exhaust to be weaker than the other three. I’ve heard very noticeable changes in how square a particular engine ran depending on the amount of lubrication on the slide valves at that moment.
Proper valve timing is always a worthy goal, and often pays large dividends, but combinations of the problems listed above can lead to a lot of man hours being put into very little return in improved operation. Like so many things in life, often a compromise has to be reached between idealism and practicality.