My club has 6 of these and low speed performance was excellent out of the box but anytime I tried changing anything other than decoder address they screwed up (been awhile so I don't remember how) and the only way to fix was a reset using CV8=254 "Reset All CVs Even If Decoder Locked"

Wow!  I spent a couple of hours setting up this loco the way I want it to operate.  I'd sure hate to just go back to factory settings.  I've been reading on some other forums where low speed is an issue on many BLI Paragon 2 products. 

Thank you all for your input.  I have tried going back to factory defaults with minimal success.  While the engine is a bit smoother, the lurching is still obvious.  My second, identical SW7 has the same issue. 

The issue you describe seems to be quite common with BLI SW7s.  On one layout where I operate two of them were put into service as yard power.  The owner could never resolve the poor operation at low speeds and eventually got rid of them.  (Note these both had the stock decoders replaced, and the problem continued.)

Might be a silly question, But are they set on 128 speed steps??

If you like the engine and are up for a decoder swap, try a loksound or zimo decoder - they have some of the best motor control out there.

The issue you describe seems to be quite common with BLI SW7s. On one layout where I operate two of them were put into service as yard power. The owner could never resolve the poor operation at low speeds and eventually got rid of them. (Note these both had the stock decoders replaced, and the problem continued.)

Now that’s interesting.

Upon hearing this, if you'd like to really get to the bottom of this, I’d do some testing. I’d take the shell off one of the locos and remove the decoder/motherboard and temporarily hotwire the loco power pickup leads to the motor. Then I’d set up a test track loop on a table and run the loco on straight DC with the shell off. Does the loco run smoothly through the speed range from stop to top speed? Or does this jumpy speed behavior continue even on DC?

If so, then it’s the motor and/or the mechanism. I would bet it’s the motor, which means a remotoring is most likely to solve the problem.

Also, make sure the mechanism is smooth ... disassemble things and look for any imperfections in the gears, etc. Try removing the motor and rolling the loco by hand, carefully feeling for any “catching” while rolling the motorless mechanism around. You can also use the pearl drops trick — clean off the gears and then put a bit of pear drops teeth polish into the gears and run the loco on the test track loop for 30 minutes each direction to polish the gears.

Clean out the pearl drops completely and then relube the loco. That should smooth out any gear action.

The other thing you can do is try a decoder that has good back EMF as well as the normal momentum setting. The back EMF literally reads the motor load and compensates if it lugs at all — this all happens with millisecond speed. With a higher back EMF setting, I’ve been able to get a switcher to run like silk through a yard ladder, greatly enhancing the sense of mass because it “floats along” acting like the frogs aren’t even there.

For those who have not worked on BLI engines, for the most part BLI designs decoders with nonstandard connector arrangements (Not NMRA or NEM compatible). Therefore to install a different decoder basically requires rewiring the engine. To to test BLI engines on DC power or with different decoders without rewiring I ended up making test cable / jumpers. Some decoders manufacturers have made decoders which were designed to swap into BLI engines, but I can not identify any that are designed to swap into SW7s. At the moment I can not get to any BLI engines to supply pictures, but you can get a sense of the complexity in the following video.

Thank you all for your input.  I'm going to keep working on these engines as time allows using your suggestions.  The DCC forum notes that the break-in time may be required. 

For what it's worth, the best description I've heard is it appears that the PID settings are causing the engine to overshoot it's speed step and have some trouble finding the right pace.  The decoder is set for 28 steps. 

Almost all of my engines other engines use Loksound decoders, so I'm not opposed to swapping.  I love Loksound's fine motor control  That may be one of the reasons I'm being so picky here! 

If I find anything useful, I'll keep you all posted!  Thanks again for the advice. 

Randy

Dear Dragen,

It may take some effort to identify the equivalent Kp and Ki CVs in your Paragon2 2 decoder,
but if you can find and program them, 
it might be worth trying the PID tuning routines outlined HERE

https://sites.google.com/site/markgurries/home/decoders/decoder-motor-tuning/soundtraxx

Yes, I know it's based on experience with SoundTraxx decoders,
but PID loop control is essentially common at it's core,
and these may help you "tune" after you've confirmed the "properly run-in" mechanicals are solid...

FWIW, in one of my cases, the "Bruce Petrucca" procedure turned a rather uncared-for 2nd hand Athearn BB "gold side motor" SD40-2 into a "1 sleeper/tie per minute" crawler, using a N scale "Ath OEM FP45" Tsunami1-variant decoder... :-)

Happy Modelling,
Aim to Improve,
Prof Klyzlr