Tigers Lair's explanations are correct and well documented.
The job is rather easy to do but be careful, screws are tiny. Apart from a very little screwdriver, you need a spanner wrench to turn the curtains screws.
I had a similar trouble with a ZENIT 3M camera; I gave an additional half turn tension to the second curtain of the shutter and since, no complain!
A tip: when releasing the curtains tension, count the number of turns; it might help you to decide if extra tension is possible or if the shutter needs first to be cleaned and lubricate (another story...). Here again, Tigers Lair is correct when he warns against over-tensioning.