I just happened upon this conversation and am so happy to have done so.
I work with full kodiak leather on an Adler walking foot machine. I have B-139 Sunguard thread that works wonderfully in my machine, however my supplier doesnt have a colour match for my dark brown leathers. I have been using 207 nylon bonded thread, when I back-tack my topstitching there will be a small ball every once in a while on the bottom. Does anyone know why this is happening? Is it because I am using nylon instead of polyester? I have contacted A&E awaiting a reply if they have something similar that will work, awaiting reply. I am new to leather work - any advice is much appreciated.
