The tension is wrong. The top looks fine, but the bottom is loose....

the needle thread tension is too loose
tighten it up so that it pulls the bottom thread up into place

Check your upper threading. Make sure it's between your tension discs. Remember loops on bottom, something is wrong with upper threading/tension. Loops on top something is wrong in the bobbin area (like the bobbin being in upside down or a loose bobbin tension spring)

start by rethreading the top thread with the presser foot UP.

try adjusting the top thread tension just a little and then sew on a scrap piece of fabric till the bottom stitch look correct.

Loose on bottom is usually caused by too loose tension on top thread, check top tension and thread is in the tension device.

Check your top tension. It sounds like it may be too loose or the bobbin tension may be way too tight. Most likely it is the top tension that is incorrect though. The TOP tension effects the bottom side of the stitch and the bottom tension effects the top. If the top tension isn't tight enough it won't draw the thread up through the top side taunt enough and the excess underneath makes a big loopy mess.

The bottom stitch is affected by the TOP thread tension :) First go back and make sure your bobbin tension is correct since you adjusted it. It should drop slightly when held suspended by danging it by the thread. If it reels out to the floor it is too loose. If it doesn't drop at all it is too tight.

Now that you have that adjusted look at your top tension. If it is loopy on the bottom side of the stitch the TOP tension is too loose. Increase the top tension.

Now take some scrap material and set machine to a wide zig zag, medium length stitch and sew a bit. Compare the top and the bottom sides of the stitch. They should look identical. Remember the TOP side effects the bottom and the BOTTOM tension effects the top side. Adjust accordingly until they look identical with no loose threads or loops and no puckering (puckering means it's too tight).

If the bottom looks loose then this is usually a problem with top threading so rethread the machine and check that the thread has gone through the tension discs. Lower the pressure foot and pull gently on the thread just before the needle and feel if it is under tension. Is the top tension device set to the middle number setting? usually 5? Unthread the top path, turn the dial to 0 and run the edge of a piece of fabric through the tension discs to clean them. Thread shreds particles of dye and fine thread pieces as you sew so this area needs cleaning regularly.

Also a good idea to check the bobbin thread is under tension correctly and that the bobbin is in the correct way. On my Janome the bobbin thread winds off so that the bobbin turns in an anti-clockwise direction. Mine has a clear bobbin cover with a threading image to remind me of this but check yours and the manual if you have it. Janome bobbins are same top and bottom, usually clear plastic so it is easy to get it wrong sometimes.

Check out www.sewing.about.com for some good general advice on sewing machine threading and maintenance, it pays to always clean and maintain your machine regularly, use good quality threads and change your needle for EVERY project, don't try and reuse them.

Of course, if you know all this and have tried it already, then perhaps the top tension device has actually failed, if turning the dial up higher makes no differerence at all, then could be mechanic time.

Welcome to FixYa!

Loose bottom stitches come from the top, not the bottom. Seems weird, but that's the way sewing machines work.

The dial tension should be set at the mid range and make absolutely sure that you have the machine threaded correctly. Both things will cause a sloppy bottom stitch.

Thanks!

If you find my solution helpful, I'd appreciate a FixYa rating....

If the top stitch is loose, then you need to tighten the bobbin tension: if the bottom stitch is loose, tighten the top tension. Hope this helps. It could also be the size needle and thread you are using for a particular fabric. Different fabrics require different notions.

Sounds as though you've misthreaded the upper tension. The best way to verify this would be to thread your machine, lower the presser foot on to the material. Now try pulling the thread through the needle. (Assuming your upper tension dial is set to a number greater than one). You should feel resistance. Should you not feel resistance, rethread your upper tension.