If you can play dueling banjos on the belt, it's too tight.
If the motor groans and whines and barely runs, it's too tight.
If the motor gets hot and looses power, it's too tight.
The rule of thumb for belt tension is: The belt needs to be just tight enough to not slip. No tighter.
I've found:
>that most black V-belts will feel loose when they are just right.
>that the amber / orange cogged belts need to be tighter than the black V-belts. They tend to slip more.
>that sometimes the amber / orange cogged belts will not work on some machines. You simply cannot get them tight enough.
>that the amber / orange cogged belts tend to slip more than the black V-belts.
>that newly manufactured belts seem to run a bit smaller than the old ones. Sometimes from stretching, sometimes cos they are shorter.
>measuring the belt length with the string method doesn't always work. Like you said it measures the bottom of the pulleys, not the top where the wide part of the belts ride.
Joan - Sabine,
Did your machines have a belt on it when you got it? A good sewing machine shop should have a belt measuring tool, mine does. It's a miniature tool like the car parts places use to measure engine belts.
Joan,
Is that motor spring loaded?
From your picture the belt does look too tight to my eyes, but it's hard to tell cos I can't reach through the screen to squeeze it myself.
I wish I could help more, but I have no idea what belt that machine might take. I checked on the Sears parts site and they don't carry that belt any more.
It's gonna be a case of try and test till you find one that works. And it might not be the one you think it is.
Joe