Jumpy loco....

[TinGoat]

Let me start by stating that I don't really know what I'm talking about...

Having said that, I've just read a short DCC CV tutorial http://www.sprog-dcc.co.uk/cvs.htm

If I understand the theory, you have:
Vstart set at 20
Vmid set at 50
Vhigh set at 100

Assuming that the DCC decoder is functioning properly...

With 28 speed steps, you have 14 small steps between Start and Mid and 14 large steps between Mid and High.

So, that means that the last step between Start and Mid is small and then the next step from Mid to High is big. That is presumably where you are getting a noticable jump.

Start by turning off, or neutralizing CV3 and CV6 by setting them to 0, and you will be able to determine if it is a DCC or mechanical problem.

Set CV6 to 60 or 0. This will either put Vmid in the true center of the speed curve, or else turn the CV6 off. Either way, the speed curve will be a straight line from 20 to 100.

If it is still jumping at the mid point, then there may be a mechanical issue. Cleaning and lubrication will likely help.