Zhenya, for more info you can check out www.graflex.org
- particularly the forums (http://graflex.org/helpboard/viewforum.php?forum=1&6593). I'm sure you'll find some valuable info there. Use the "search" link and enter "focal plane tension" and check "search for all of the terms" - you'll find the info you need.
If the negative came out underexposed (and the development was OK), it would mean that the curtain is faster than it should be - which would be relatively unusual. They are usually SLOWER
Anyway, it's possible that the previous owner "serviced" it, and turned the tensioning spring a couple of turns too much.