The rifle is not mine, but my friends.  He bought it at a gun show in the fall.  It is a black synthetic folding stock chinese SKS.
The bore is in good condition and all the parts SEEM to be in working order.
But it has a problem.  It will not cycle properly.  I have experience with the SKS rifles and first thought it to be a problem in the gas system.  I took a gas system from a working SKS and replaced the piston, gas piston extension rod and spring.  I went to fire, and it had the same results.  So now i am a bit perplexed.  Here is exactly what it does. Tomorrow i might be able to get a video if that would help.
It chambers a round fine, fires fine, but the bolt only comes back say  1/2 to 3/4" from its forward most position.  The bolt doesnt cycle as smoothly by hand as most yugo's i have fired, but is on par with other chinese.
What needs to be replaced? or should ym buddy just take it to a gunsmith and let him work with it?

thanks in advance
-karaya

Couple things to try given the aftermarket stock.  First, is it fitted correctly in the stock with no obstructions.  Second, reexamine the gas system.  Is there grease or accumulated gunk blocking the gas port?  Is the op rod, the thing the gas piston pushes, free and clean.  There is just not much that can go wrong with these and its usually cleaning issues.  I had a paratrooper some years ago that sat unfired for 10 years.  I have no idea if it goes bang or not and I had never cleaned the internals when I sold it.

The gas port is clean, the op rod is also clean.  The gun is spotless, everything is clean.   I will remove and refit the stock right now.  I will be taking this gun back to the range tomorrow and will keep you posted.  Thanks for the advice!

Same problem.  I dont see any thing that could jam the bolt.  I still think it must lie in the gas system.  The guns #'s are all matching, so it must of worked at one point.  I think maybe a replacement of the piston, and piston extension might do it.  Damn, i wish it would just work!