I recently took my Elna Supermatic out, asking myself why I don't use this wonderful machine more often... and alas after a bit of sewing, I seem to have remembered.
When I took it out of storage, the top tension was set at a bizarre extreme (either very loose or very tight, I forget which). I oiled up the machine, adjusted the top tension, and it calibrated at a more expected intermediate setting. It sewed beautifully for a while, then for some reason I can't fathom, suddenly the top tension was way too tight.
I can't get the top and bottom tensions to balance except at an extremely loose setting, and that leaves both the top and bottom stitches really loose.
I tried unthreading and rethreading the machine, both top and bottom. I tried switching to a different bobbin (from aftermarket to OEM). The machine is well-oiled.
I do know that when you insert the bobbin into an Elna, you have to put a finger on it (left hand) to hold the bobbin still, then pull the thread to the right until it clicks into the tension spring. Failing to do that would leave the tension too loose, but I don't think that is my issue.
The fact that I had put it away (frustrated) with a weird tension setting on previous use reminds me that this probably isn't a new issue on this machine.
What else should I check? Any ideas for machines whose tensions inexplicably phase-shift?
EDITED: The next day I unthreaded and rethreaded the machine, and it sewed beautifully. I suspect the bobbin thread had in fact come out from the bobbin tension spring, causing loose lower thread, and I just hadn't noticed it; and if it's not the bobbin spring, then the culprit causing tension wonkiness is somewhere down in the bobbin area. I'll just have to sew more with this gorgeous machine to figure it out.