Lets talk about tensions. When a machine comes into the shop, no matter what the problem is, the customer will usually say, "it's the tension." If the timing is off, "it's the tension." If there's a burr on the hook, "it's the tension." If the needle is in backwards, "it's the tension." You get the idea.
I'll do a run through on how we balance the tensions at the shop.
Before we can work with tensions, we clean and oil throughout the whole machine. It's especially important to clean and oil your bobbin area before working with the tensions. That's something you should be comfortable doing yourself.
When you thread your machine, the presser foot has to be in the up position, to open the upper tension disks. If the presser foot is down, the tension disks are closed tight, and the thread can't get into them. When that happens, your fabric will have loops on the bottom, like this...
You can click on the pictures to make them bigger.
Next, make sure there's no lint in the upper tension disks or inside the bobbin case. You can sweep the lint out with your lint brush.
Then set your upper tension dial on "normal." If you don't have a "normal" setting marked on the dial, set it at 3.
Correctly thread your machine, top and bobbin. Put a light colored thread in the bobbin and a medium to dark colored thread in the top.
Set your machine to do a medium zig-zag (unless your machine is just a straight-stitch machine). On most machines it's 3 on the width and 2 on the length.
Sew on a light colored, good quality, cotton fabric for about 6."
If the bobbin thread is pulling to the top, tighten the bobbin case tension by turning the little screw to the right. Remember, "righty tighty, lefty loosey." Here's some pictures of bobbin case tension screws...
If the top thread is pulling to the bottom, that's what you want it to do. But, you only want the top thread to be a "tick" on the back. If it's really pulling to the back, loosen the bobbin case tension by turning the little screw to the left.
After working with the bobbin case tension, if you're still not sewing well, try adjusting the top tension. It works just opposite of the bobbin case tension. If the thread is still pulling to the top, loosen the upper tension. If the thread is still pulling to the bottom, tighten the upper tension.
Here's my nearly perfect sew-off sample.
If you're still not sewing well, it may not be the tensions, and you may need to take your machine to the shop.