There is a really good repair manual tells how to trouble shoot and if needed take the tension apart and put it back together. I have given up just adjusting. I just take the tension apart and put it back together. If all the parts are there and the spring is right it adjusts in a VERY short time. Here is a copy of the manual.
http://www.tfsr.org/pub/technical_in...echanism_2.pdf
You will have to figure out what instructions apply to your machine.