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One more thing that can affect the chuffs you hear is whether the valve gear is in Full ("in the corner") or backed off to nearer the "Company notch". When the valve gear is backed off this way it is possible for the "D" to not move far enough to actually admit steam into the cylinder and that will produce fewer chuffs per revolution. On my Mikes, I can get it down to just one chuff per revolution... i.e.: the loco is actually running on just one end of one cylinder, which at very slow speeds can produce a very jerky motion to the train... I have had the tender connection slip loose such that the train is being towed by the rubbery hoses instead of the drawbar and when running slow on one end of one cylinder the train can look like an "inchworm" or some animated cartoon "choo-choo".

That is a good reason for not hearing all 4 in a strong staccato beat, but in this case it is also related to the quality of the sound found on youtube. Try listening to it in a high resolution mode, which is stereo sound compared to the normal split channel mono sound waves.

Company notch should not cause one beat per revolution, and by running the engines like this, you are causing adverse wear and tear on the valve gear. The same applies for full-size locomotives, all of them have a cutoff ratio, normally around 75% from neutral. The mikados have three notches in either direction of travel. When running, only the second two (from neutral) should be used on a properly timed locomotive. The notch closest to neutral is useless since it is past the acceptable amount of reduced travel the valvegear can support. As you said, notch one (from neutral) causes a severe jerking motion, just imagine the stresses produced on the valvetrain!

Mark, once the locomotives start to accelerate, they are usually notched up, which retards the valve timing so that the pistons have a cushion of steam at higher speed. You are having trouble distinguishing the beats at a higher rate of speed, once the staccato reaches a point where the strongest side of the locomotive (yes they are power biased, even the best running engine will have a stronger side) your ear can only detect the stronger beats. It is also related to stack geometry and how much velocity of steam is allowed through at a time.
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