Rhythmic stoppages
Does anyone find they get rhythmic stoppages or rough running on their loco at longer period than the wheel revolutions?
My 6th restoration job has turned difficult, due to a Mallard with a strange mis-step every 4th revolution. At low speed or high speed the loco pauses briefly once every 4th turn of the wheel.
Loco top speed is greatly reduced.
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My guess would be that one of the tiny O rings between the valve block and cylinder block is compressed and partially covering the steam passage, or debris from a deteriorating valve pressure pad blocking a steamway.