Are you using a modified bobbin case for sewing as you mentioned using 15-1 needles? The original needle is a bit shorter so using the 15-1 needles can cause damage to an unmodifed case especially in zig zag stitches.Typically people either modify the bobbin case or use an alternate bobbin case that allows a large enough opening to allow for the longer needle depth. Apparently you do not have to change the timing. I recently got a Singer 319w which is the flatbed version of your machine.