When I have trouble with a sewing machine tension I usually fight with it until I pull out the bobbin case, clean it, then adjust it the way it says in a number of places.
Basically I put a full or nearly full bobbin in the case and hold it by the thread. If the bobbin drops away of it's own weight, it's TOO LOOSE so I tighten it.
If it just hangs there and doesn't drop away I give the sting a sharp short snap. If the bobbin case does not drop away a little bit, it's TOO TIGHT, so I loosen it a bit and repeat the test.
When the bobbin case will drop a bit and stop then I put it back in and work on the top tension.
Invariably when I've done this I'm able to get the tension squared away. I just did exactly this last night on a Japanese 15 clone. Nice stitches now.
The vertical loading Singers can be set up the same.
Just remember that sometimes you have to completely remove the top and bottom tension assemblys, take them completely apart and clean them with something that will remove old gunked up oil. If the tension parts or disks look the least bit amber in color, they are really needing cleaned. That amber color is old oil. It will drag on the thread and mess up the tension.
Joe