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I purchased a new Hornby K1 loco and put in a Hattons 8 pin decoder. It runs OK forwards, but in reverse it either sticks or runs intermittently in small jerky movements.

Is this likely to be mechanical (nothing obvious) or the decoder?

I would normally swap the decoder to test but as a newbie I don't have a stack :-)

That sounds like a mechanism problem. Take a look from the underside for anything visible such as side rods which might catch on
each other, or on fixed structure. (Incidentally, this is a check I perform on every new outside valve gear model. Modern drives can
screw a caught rod into a ball at relatively slow speed before you can press the stop button!)

Do you have any way to run the model without decoder on DC? Even a 9V PP9 type battery or other low voltage supply will do.
(That's another of my articles of religion in model railway, thorough DC test ahead of decoder fitting.)

FWIW. My overall assessment of the K1 is that the mechanism is very good (based on the four examples I have seen) so if the
trouble proves to be something you cannot solve and the model is new, return for replacement, pretty likely to get a good one!
Yes it was mechanical. Not sure what, but after I had a little play and manually pushed it backwards at very slow speed it is working albeit it not as smoothly as I would like. One problem in using Hattons from NZ is 'returns' are costly!
Do you have a continuous run that will now enable the model to be 'exercised' for a couple of hours at a modest speed to see if it
will settle down to running properly smoothly? (Ideally alternate forward and reverse regularly, 15 minute intervals or thereabouts.)

This may be nothing more than some grease 'blobs' in the gear train. The viscosity of grease can absorb a very large proportion of
the circa 1W power output of the motor, and because the worm drive shifts the pinion from side to side with forward and reverse,
the whole gear train slightly changes position, which can result in different drag forward and reverse. What you want is the grease
to be 'worked' until there is a thin and even film on the working faces of the gears, and any blobs smeared out so that they are not
causing significant drag.

If there is some other mechanical fault (split gears do occur) best to surface this early while the model is a new purchase. Despite
the expense and inconvenience, at least the parts replacement side is covered.
DC Marvel
Check the quartering on all sets of drivers, one axle may be very slightly out.
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