I actually have a Husqvarna 901 with threading issues

Question edited for full 'make model (what it is)'.
Question moved to maker category.

You either have a threading issue, or a tension issue, or both issues. Read the Article and watch the videos.

By thread tension, we mean the amount of thread that can pass through the machine to create the stitch. The more thread in the stitch, then the looser the stitch. The less thread, then the tighter the stitch. Top thread tension is controlled by a dial on the machine's thread path.
Sep 11, 2018

Adjusting tension on sewing machine - Artisan Stitch

FWIW, I was unable to find any information for turning off the thread portioning function. Unfortunately, this appears to be the thread tensioning system, so disabling the thread portioning would be disabling the thread tensioning--something that is critical to sewing. It appears this model may have some issues and perhaps this one is particular to your machine. It may be best to take it for service.

Perhaps this will help:

Husqvarna Viking Designer Ruby 25 Tension Thread Portioning Deluxe Stitch...

Husqvarna Viking Designer Ruby 22 Bobbin Thread Sensor

Husqvarna Viking Designer Ruby Review Sewing Insight

..

Your dealer should keep a record of what was done to your machine. Actually, they should give you a record of what was done to your machine.
Calmly let them know that it used to do this and now it won't. Maybe they can take care of the problem for you.
If they are not willing to help, take all of the repair documentation that you have on what was done and contact another Husqvarna Viking dealer to see if they can help.

The picture diagram works the best for this, it tells you how to tread for every different stitch that you want to perform. You have to thread it in different ways for every stitch so it is difficult to tell you in writing. If you visit your local Viking dealer they would be able to make you a copy and I'm sure show you how right there.

Suggest you contact an authorized Husqvarna Viking dealer. They can order you a user manual. Authorized dealers can be found at the Husqvarna Viking website - http://www.husqvarnaviking.com/us

Brief description of probably how to thread from looking at a picture of a Classica 90 machine which should be similar -

Thread sits on spindle on backside. Pull thread straight across to upper left backside and fit in the metal guide. It should pull easily with only a little resistance. Pull thread forward across top toward you. Let thread fall between the metal disks (doesn't matter which one if more than one) and pull thread straight down. Pull thread slightly to the left to fit under a hook of some type and back up to fit through the large hook that moves up and down when the handwheel is turned. Come straight back down to needle and thread through eye of the needle. Just above the needle should be a thread guide (sideways hooking looking metal piece), slip the thread through it to hold thread close to needle. Hope this helps. Otherwise a local authorized Husqvarna Viking dealer will be glad to help you. Good luck.

you need to change your needle first. then if that does not work you may have a small area on your bobbin case that has been damaged

try threading the bobbin by lifting the presser foot, and pulling the thread under the foot(critcal) over the bobbin winding knob (metal piece) you now use, and to the bobbin winder, It touches more places like this and winds more evenly and snugly.

This could either be due to the bobbin being wound too loosely, or the tension on the top thread is too loose. This just happened to me the other day, and I thought it was the bobbin thread, but then I put another bobbin in with a different color than the top thread (this is a huge help in diagnosing thread issues), and I was surprised to find that it was actually the top thread that was a big loopy mess on the underside of the fabric. I fixed it by tightening the tension on the top thread (quite a bit), and everything was fine.

there is the temp fix-epoxy when putting it on (actually can work well, just dont epoxy it on the bottom and freeze it or the permanent fix -new hook cover complete where the cog is'nt going to come off on the new part

Sometimes the thread tension needs to be cleaned out. I put a smal screw driver in where the silver tension guides are (between the dials) and open them up and wiggle them a little bit.
If the machine is cutting theads and or not catching threads on the back the timing may need to be adjusted.

What type of stitch are you trying to do?