Don't just clean the gas port clean everything,it all works together. The Yugo M59/66 SKS rifle probably give more trouble than any of the SKS rifle as far as FTE's fired rds.
1. Make sure the gas valve selector switch is all the way over to the right for semi-auto shooting,straight up is for single shot and grenade launching.
2. Try differ brands of ammo,my Yugo would eject anything I ran through it but Wolf. After I replaced the valve even the Wolf worked.
3. If you suspect that you have a leaky gas valve you can make a temporary gasket from a paper clip and try it with the ammo that fails to eject.
Yugo Gas System Fix
Replacement gas valve