The gas shut off (you know, the one that you switch when you want to shoot grenades) on my Yugo SKS doesn't want to move at all.
When I take the gas tube off the gun, then I can move the gas shut off valve easily.
But the gas tube - gas valve interface is very tight... and quite honestly, it seems it is impossible to push the gas valve into the gas tube a quarter inch (when locking it back to fire grenades) and even rotating it for that matter.
What's the deal? Obviously I'm not going to fire grenades but apparently I cannot flip the switch without removing the whole gas tube... which is very uncool when you want to show friends the grenade sight.
I had this problem on one of my Yugos. It was a build up of carbon in the valve assembly. I soaked the crap out of it with solvent and Kroil and now it is working again.