Just recently, I had a steam loco that used to run fine but over the last two years it has run more hesitantly. It had a tendency to slow down at a certain point in the driving wheel rotation especially at low speed. When I have encountered this before I found that one or more of the driving wheels were out of quarter. If you examine the driving wheel of a steam loco, you will note that the wheel is offset on one set compared to the other. This is essential for operation since you need a driving force pushing the wheel round at all times. Sometimes these wheels get strained and the one wheel on one side of one set can slip just a fraction. If this happens to one set (particularly the main driving wheel) but not the others the change can set up a tightness in the connecting rods at certain points in the wheel rotation which cause the motor to strain at certain points in the rotation causing it to slow down, then speed up again as the tightness frees.
What to do? Requartering is a pain, requires resetting the correct angle on the wheel then making sure it doesnt repeat by some form of repinning the wheel. Expensive, time consuming and most people dont do it.
Wait are you sure it needs requartering? Coming back to my story the answer may be no. I discovered on my loco that the slowing down in rotation was caused by dried grease in the gearbox mechanism – probably on the first gear on the wheel since it exhibited the same symptoms as the out of quarter situation. I admit I had to strip the motor assembly down and remove the connecting rods to diagnose the situation BUT cleaning it out and reassembly was a lot better than having to requarter!
Next time you think “oh no, its out of quarter”, be digilent and check it all first!