This is an old thread, but I have an LS2-F52A, so I thought I would comment. I bought it new around 1996, for sewing marine projects (hobby, not as a business). In the last couple of months, I started playing around in the leather craft world. I've been trying to use 138 thread top and bottom. It's not working well. I can get the tension adjusted so it sews nicely when going forward. However, a back tack is pretty ugly on the bottom.
One problem I noticed was that it would sometimes "short stitch" when going forward from a back tack. Focusing totally on the movement of the leather, I saw that the leather moved a little backwards as it made a stitch. Thinking about it, the only part that moves backwards (other than the needle when it's out of the leather) is the feed dog. This only happens for the first few stitches after a tack tack. I began to wonder if the feed dog was catching on the jumbled stitches from the back tack. So, I lowered the feed dog height slightly, and the problem went away. HOWEVER, it's now somewhat finicky when removing the bobbin, because the feed dog linkage interferes. You have to go about an inch past the hand wheel alignment mark, and then fiddle and wiggle to insert or remove the bobbin. I'm beginning to think this really isn't an optimal machine for leather craft. Maybe thin garment or upholstery, but not for what I would consider "leather craft".