Answer
Sep 05, 2025 - 01:15 PM
If your Bernina 330 is sewing fine on top but the bottom stitches are getting loose after a bit, that usually points to a top thread tension issue, not the bobbin. I know it sounds backwards, but loose bottom stitching typically means the top thread isn’t staying tight enough.
Here’s what you can try:
- Re-thread the machine completely — even if it looks fine. Make sure the presser foot is up when you do it so the tension discs are open.
- Check the top tension dial — try turning it up a notch or two and see if it improves.
- Make sure the thread isn’t catching on anything — sometimes it snags on the spool or the little thread guide, especially with certain thread types.
- Clean under the needle plate — lint buildup can mess with stitch quality over time.
- Watch when it loosens — if it happens after a few inches, it could be that the thread is slipping somewhere or the tension is gradually backing off.
Even if your machine tech says it’s sewing fine, these kinds of issues can show up under different fabric, thread, or tension settings.
