Oops, posted this in the Chinese area then found out this may be the type of work for you! I just bought a used 1967 Chinese Arsenal 26. Everyone on the range (with some SKS folks) thought it looked in great condition and were very impressed with the cleanliness. I took it all apart, except for the trigger assemebly and spend some good time on it.
No mods to the rifle, except the bolt serial number is FAR different than the rest of the rifle.
Here is what happens. You load 10 rounds. Let the bolt go forward and it cleanly loads the round. Safety works great. Trigger pull feels to good to me. Squeeze the trigger and the round fires. Brass is ejected, and a new round loads nicely. Then I goto pull the trigger again and.... nothing. The trigger feels "dead" and not engaged at all. I can slightly pull the bolt back and either, reload the same round, eject that round and load the next one... and then pull the trigger... it fires, brass ejects, new round is loaded... dead trigger AGAIN. Tried different ammo. Looked all over for dirt, crud, cosmoline. One of the SKS guys on the range completely took it apart and feels that the bolt in semi-auto is pressing down on the ???? (release, level? whatever connects to the trigger) too far. He showed me where if this item wasnt pressed far enough it wouldnt engage, and if to far it wouldnt. You have to be somewhere in the middle.. so, does this sound like a job for you? He said a few hits with a metal file may do it, but I dont want to mess this up.