No tension on snowblower pull cord?

The pull cord on our Toro snowblower (older model 521, year = ?) only recently stopped having tension when pulled. I did read that it might have to do with some sort of lubrication, so I gentley tapped it with a hammer and got the tension back. It has now happened again. Is there a lubricant that I can buy to help with this problem? Is this something I should handle myself or take to the shop. I was told not to mess with the spring. Thanks in advance.