Tension,

[AlZilla]

Disclaimer: I'm a total novice with sewing machines and my comments are worth exactly what you paid for them.

I haven't had one get "loopy" due to tension.  But if the top thread is showing on the bottom, you could have too little top tension or too much bottom tension.  And vice versa. It's all about balance.  I have had a case where I couldn't quite get the top thread to stop peeking through when sewing a single layer - but sewing 2 layers, it disappeared into the fabric.

I have a nearly brand new Brother domestic that was getting loopy on the bottom (with top thread). Turned out that on that modern machine, since the tension arm is mostly enclosed in the case, I wasn't quite getting the thread well into the end of the arm. It would loose the thread from the end of the tension arm (which I couldn't readily see) and caused no end of trouble.

For me, anything the thread touches (top or bottom) has to be clean and maybe even polished. If you're seeing big loops, it seems like more than a simple tension issue.

I'm following this one to learn and admire your determination.

[DonInReno]

This chart is only accurate if the needle is large enough to allow the knot to be pulled up in the material.   
 

For 95% of all sewing the bobbin tension isn’t messed with other than the initial setting.   With the bobbin in the bobbin case and holding just the thread, gravity shouldn’t let the bobbin/case fall.   If the thread is then giggled up and down gently the bobbin should start to pay out thread.