I just bought a 3160 and have noticed the zig zag stitch has a pucker down the center. I took it back and the tech messed with it for a while, did some adjustments, and told me all Janome machines do this. Can anyone confirm this?
I sounds like a top or bottom tension may be set a little high. Depending on how light your fabric is, using a washaway stabilizer may help prevent that pucker.
ZigZag is not all that good on anything that is a soft fabric and you definitely would do better using a wash away stabilizer, or starching the fabric a bit.