Hello Redsquirrel -
That's interesting that you say you found your 201 the best. I have a 201K treadle that I still haven't tried. because my hopping foot was not compatible. What sort of foot have you been using? Even though I can use my 15K for all the FMQ I do, it would still be nice to give it a whirl on the 201K.
Hello Mdaniels -
Here is a link showing how to drop the dogs on my 201K. I imagine it will be the same on your 15-90, although I don't know. Mine is a 15-80, the dogs don't drop, so I work with the stitch length at zero.
http://lizzielenard-vintagesewing.bl...nger-201k.html
Hope this helps.