Top thread is tight but erratic bottom one is loose and uneven

Suspect uneven feed of top thread.

Manual for reference if needed.
https://www.brother-usa.com/-/media/brother/product-catalog-media/documents/2018/04/20/08/38/um_pc_8500_en_194.ashx


Please check the following;

• This machine has a horizontal spool. Try using a spool cap that is slightly larger than the spool. Verify spool is in proper orientation, and threaded per instruction. See page 19.
• Embroider foot "Q" is installed per manual instructions. See page 32-33.
• Feed dogs are down (supposed to happen when automatically table is installed but check and make sure).

There should be no looping of thread on bottom side. This would indicate top thread tension is too loose or uneven feed (spool cap and/or orientation). Suggest you preset tension prior to installing table using normal presser foot and single layer of same fabric and same thread. Select a straight short stitch (machine default) and tension 4. With a single layer of fabric, tension at 4 is likely high and you will see bottom thread loop on top side so slowly reduce tension until bottom thread no longer shows on top, Then switch to embroider setup and test run.

Comment below with results and I'll assist further if need be.

First, remove the bobbin and clean the entire area; if your machine model has a bobbin case, be sure to remove any lint from between the overlap. Change to a new bobbin in case the bobbin is wound poorly (happens even to pre-wounds).

Check thread paths of bobbin and top threads and be sure tension isn't too loose in bobbin or top; sometimes the auto tension changes erratically when stitching on thick fabrics, check your manual for resetting it to default or for making adjustments.

Be sure the needle is one that is the right size and type for the fabric you're stitching on and use a new one; bent needles happen easy, and a slight, almost undetectable bend will cause this type of issue.

normally loose bottom stitches indicate a problem with the needle thread tension not the bobbin. try removing top thread completely and then rethread machine with presser foot down, if you do it with foot raised auto tension will not function

try adjusting the top thread tension one number or letter higher
sewing on a scrap piece of fabric till the stitch looks normal.

If you have the instruction manual, you might read the chapter which describes the proper adjustment for threads and fabrics.
If you do not have one, try to find it on eBay. It is the huge source of everything.

This is a problem with thread tension. The needle thread is too tight and the bobbin thread is too loose. I don't have one of these machines, but there should be either screws or knobs to adjust. Look at your manual. Here's a page I found on bobbin tension.
https://www.manualslib.com/manual/1022927/Toyota-Quilt-50.html?page=50

Hello

Load your Kenmore sewing machine with the right thread and bobbin thread for your project. If you use the wrong weight thread for your fabric or severely uneven weights of thread, this can cause the appearance of a tension problem when it is, in fact, a thread weight problem or a problem with your sewing speed. If you go too fast, you will break the thread. If you go too slow, you can cause loops. Uneven pressure can cause stitches to vary greatly.

Test your Kenmore sewing machine's tension by sewing on a piece of scrap fabric. If your tension is way off, then test on some basic cotton or a firm blend simply to get your tension right. Do not try to start adjusting your tension on silk or leather. Get a base adjustment on a more standard fabric first.

Look at the top threads and bottoms threads. If the tension is off on one side, it will usually affect the other side slightly. So select the side that looks more maladjusted to start adjusting the tension.

If the top thread is too loose or too tight, you will need to adjust the tension dial. This dial is usually located a few inches above the needle on the Kenmore machine body. Usually, your machine will have markings to indicate which direction tightens the tension. If these have worn off over the years, watch the dial as you turn it. If it appears to move toward the machine body, then it is tightening.

If the bottom thread is too loose or too tight, then you will need to open the door to the bobbin case. On some Kenmore models, there is a little screw near the bobbin case that can be adjusted slightly to adjust the tension. On other Kenmore models, there is a screw on the bobbin case itself--not on the bobbin, but on the bobbin's casing. This will also adjust the bottom thread tension. Adjust the bobbin thread tension to make slight changes.

If the thread is looping severely on one side, the thread weights might be off. For instance, look to see if you have a lightweight thread on the bobbin and a heavyweight one on the top. If the thread keeps breaking, the top tension or bottom tension could be too tight or the thread is too lightweight for the material you have selected. Check your combination of threads and material. Also make sure you are using the right weight needle and the right foot for the sewing step or project you are working on.

If your Kenmore sewing machine's tension is off severely or if you have not used your Kenmore in a long time, then you might need to take your machine in to be serviced. Sometimes, the inner parts need to be oiled or the entire machine might need to be cleaned and calibrated by a professional. If you sew a great deal or if your machine sits idle a lot, both situations will usually require some servicing to keep your Kenmore sewing machine and its tension in perfect working condition.

try this:
Tension:
If the thread is loose on the bottom of the fabric, it's actually the top thread is too loose. Think of 2 little elves playing tug of war in your machine, one on top and one underneath. If you have loops on bottom, the top needs to pull harder (tighten top tension)
Thread test
Test your thread quality to start, thread your machine and LIFT the presser foot (this opens the tension disks). Pull your top thread straight back. If you feel no tension no matter how much thread you pull, your thread is good. If that your machine passes that test, we can go to step 2. If not, try different spool of thread, do the same test until you get a smooth pull (tip: always check your thread this way when you thread your machine)
Step 2:
Pull on your threaded bobbin thread while it’s in the machine. Does it pull smooth and even? If so, go to step 3. If not, try a new bobbin (bobbins get bent or distorted if wound too tight).
Clean thoroughly in the hook and bobbin case area and oil 1-2 drop is all.
Jamming bobbin case:
If it is damaged from turning out of place once, it could have rough spots on it that makes the thread hang on it, and keeps making it turn over and over. Use a finger nail board (fine sanding) and smoothall rough spots. Then reset the bobbin case taking care to put the notch in the bobbin case in alignment with the proper spot in the machine (basically 5 o’clock) when looking at the round area where the bobbin case goes

Tension:
If the thread is loose on the bottom of the fabric, it's actually the top thread is too loose. Think of 2 little elves playing tug of war in your machine, one on top and one underneath. If you have loops on bottom, the top needs to pull harder (tighten top tension)
Also look at:
Thread test
Test your thread quality to start, thread your machine and LIFT the presser foot (this opens the tension disks). Pull your top thread straight back. If you feel no tension no mater how much thread you pull, your thread is good. If that your machine passes that test, then let me know, we can go to step 2. If not, try different spool of thread, same test until you get a smooth pull (tip: always check your thread this way when you thread your machine)
Step 2:
Pull on your threaded bobbin thread while it’s in the machine. Does it pull smooth and even? If so, go to step 3. If not, try a new bobbin (bobbins get bent or distorted if wound too tight).
Clean thoroughly in the hook and bobbin case area and oil 1-2 drop is all.

can u showe me a visial on how to thread