So, if I get this straight, the ribbon is not advancing much? It is supposed to advance slowly, like a few mm at a time.

I would take the spools out (By the way, are they Royal 10 spools? Look up Royal 10 spools and see if they match) and check the worm gears inside the machine to make sure they aren't clogged with dust and stuff.

All that screw does is adjust the tension which the mainspring pulls on the carriage.  You say you turned it counterclockwise--you have loosened the tension a little or a lot--depending on how much you have turned it in that direction.  Turn your machine on its back (being careful to put a piece of cloth or an old cup towel to protect it), run the carriage to the left and right, and see what the drive shaft does.  If it spins and there is no effect on the carriage tension, set the machine back down and observe the ribbon mechanism on the front to see what it does.  On the left, you'll see an S-looking thing that is supposed to spin, along with two round wheels--one to the top and one to the bottom.  These usually freeze in place, and need to be oiled and worked with a pair of needlenose pliers.  Check also to see how the worm gear contacts the upper and lower gears.  Lastly, check the spin on the ribbon spools each side and see if the trip arms are not in the tripped position--meaning that the little wheels aren't in the same path as the essy-thing that spins (I'm not sure what this is called, but that's what it looks like).  I hope I've been of help.  Tell me how you come out.