I was going to say you need to set it on 0, I have a Singer 328K and that is what I have to have it set on to straight stitch, it also has cams to make decorative stitches. The 0 setting is for the swing with 4 being the largest swing, for the largest zig zag stitch.
I'm glad you figured it out, it looks like a good sturdy machine and looks to be in very good condition, glad you are finally using it. Good for you !!!