I don't own a model 15 my self, but I have test driven one or two. I have a 99, a 66 and a 201 and the bobbin mechanism is a bit different on thes. Even so, I dare say it's only a question of fine tuning and getting to know your machine better, it should sort out any stitching issues. Model 15 has a reputation of being very forgiving when it comes to thread and tension, but they often need a bit of tending to when they are taken out of years of storage and no use. The feed on these old machines is usually excellent.