My 1940 Underwood Noiseless standard is on my desk right now. I keep coming back to it even though the keyboard feel isn't the best, and the printing is still a bit weak, and the visibility of the typed text is atrocious. But I brought it back from the rusty grave about a year ago and it has been developing a lighter touch and better printing ever since so I guess I'm hooked on it. (But I'm rotating it out with the Underwood Touchmaster Five, and the Royal KMM, and even the R.C. Allen Visomatic, which needs a platen recovering badly)

It might be that someone has adjusted the carriage shift too "light" and the threaded adjuster is sticking up and stopping the movement of the carriage when it is resting in its normal position. Move the carriage by hand to the left or the right and look along the back wall of the case for a spring that hooks into a threaded rod and has a lock nut on the top. There's one near each corner.
   Or it could be something else entirely. Get a flashlight and sight along the gap between carriage and the rest and work the carriage back and forth with shift on and shift off. Good luck. PS SM4's are notorious for the case-to-chassis rubber grommets getting squished over time and that lets the sheet metal case "rise up" and catch the carriage.  There are lots of mentions of this problem in this forum if you do a search.

Yep, I'm beginning to develop a real envy of the story-tellers since I started using typewriters and wanted something to write. Still, I love the physical pounding of the keys. And I'm absolutely sure this daily exercise with my mind--keeping the spelling and grammar correct--is bound to be good for my mental faculties as I get older. Some old people take up knitting. Some take up whittling wood. So I like pounding the keys for my recreation. That's about the  gist of it.

Repartee wrote:

I guess the philosophy was that if you were in Irkutsk, say, and it would take three months to get a replacement spring it would keep working on the remaining one until help arrived. You want to be able to keep working in Irkutsk, so long as the vodka holds. 

  Yeah. Boy. Tell me about it! I couldn't wait for Spring in Irkutsk!

A couple of years ago I literally pulled a muscle in my back picking up an IBM model B electric and carrying it into the other room. Took three weeks to get over that one.