JohnSawyer posted this on another thread and I thought it might be worth talking about. Not wanting to hijack the thread I'll start a new one.The biggest problem I've run into has been bottom tangles when trying to do bartacks on straps.
This is one of the biggest issues raised when the DIY crowd has problems. It is important to look at how the machine works and why this is a top thread tension problem. Almost always, barring some more exotic malfunction, increasing the top thread tension is required to fix this and that is all that is needed.
Step by step
1 The needle pokes through the fabric and withdraws leaving a loop below the fabric. This loop is what the hook catches and the bobbin thread is locked into the loop.
2 The take-up lever goes up and down synched, but offset with the needle. As the needle reaches the top point the take up arm is still rising. The function of the take up arm is to pull the loop up from under the fabric tightening the lock stitch formed by the hook.
Here's the dynamics of the system. The needle pulls the thread from the spool as it goes down. The take up arm pulls the slack out from under the fabric and tightens everything up in the stitch.
If the top thread tension is too loose the take up arm does not overcome the resistance of the fabric. As a result it leaves the loop below and draws thread off the spool on its way back up. The way to solve this is too make the resistance from the spool side greater than the resistance from the fabric side.
That's why, as strange it may seem at first, the loops under the fabric are caused by the top thread tension being too loose.
End of dissertation.