With the blade installed in the bottom and top holders and no matter how tight you set the tension, the blade is still wobbly. What could be wrong?

SOURCE: how do I adjust the bottom thread tension?

some machines have a nob on the front loose is low numbers like 0 1 -2 most machines work well on number 3. If the nob is not on the front check and see if it is on the side.

SOURCE: My Bernina 930's lower tension

Hi - you might have a dirty bobbin case, or need a new one. IAfter awhile, little teeny dust bunnies can collect in your bobbin case and cause your bobbin tension to go wonky, especially if you use cotton thread. If you happen to have an extra bobbin case, try using the new bobbin case to see if that solves the problem. Otherwise.....

• Remove your bobbin case and take out the bobbin. Find a small, stiff brush like the one that came with your machine and stand in very good light. Brush out the inside of your bobbin case, and push one or two bristles of your brush into any opening you can see. If you see any trace of dust but you can't get it out, use tweezers to gently pull the dust out. Now look at the outside of your bobbin case. In the small opening where you insert your bobbin thread into the bobbin case, there is a small piece of metal with screws in it the lays over the bobbin case. Brush that area carefully, and again, insert one or two bristles under than piece of metal and into any other small opening where the bristles will fit. Try sewing again. If you are still getting loops, take out your bobbin case with the bobbin thread still inserted and grab the bobbin thread with two fingers. Let go of the bobbin case - sort of like you are holding a yo-yo. If the bobbin case starts to drop as soon as you let go, your bobbin tension is loose. If the the case doesn't move, gently snap the bobbin thread you are holding and see how far the bobbin case moves down toward the floor (you might need to try this a couple of times, the bobbin has a nasty habit of flying out). When you do the snap, the bobbin case should move down about two inches and stop. If do not know how to adjust the bobbin tension, either check your manual or take the machine in for service.
• If you haven't already, it's a good idea to remove the thread plate that sits under the needle and clean that area out with your brush. Also, get a really good flashlight and check the area where you insert your bobbin case. Gently turn your fly wheel and look inside the hook area - remove any dust or bits of thread.
  

SOURCE: Zigzag stitch pulling tight, looks wrong

Try adusting the bobbin thread instead of the top. Always make sure to insert the bobbin so the thread is pulling in a clock wise direction. Both threads top and bobbin should have about the same resitance when pulled.

SOURCE: bottom tension to tight

There's hardly any way to significantly adjust the bottom, or bobbin tension. There's just a small, flat spring attached to the side of the bobbin case, under which the thread runs. There's adjustment there. You need to adjust the top thread tension via the small thumbscrew that's against the plates on the top thread tension discs. Increase or decrease tension there until top and bottom threads are equally centered between layers of material.
You can adjust presser foot tension with the knurled nut at the top of the machine. I never had the kind of problem you mention though. I set my machine at about 6 stitches per inch or a little more (max is 5, recommended), and presser foot tension is only heavy enough to pull a single layer of material through.
Hope this helps.

SOURCE: tension problem

.There is a little screw attached to the bobbin carrusel. You can try to turn it just a little bit. The reason is that this carrusel gets loose after a while.

Clean and oil your machine at least monthly.

Hope something helps!