Now that you have the gas system apart, clean off all carbon.
The best ways to do this is to use a "lead-away" cloth to wipe it off, or even better use a carbon remover chemical.
Slip 2000 Carbon Cutter works very well. A 15 minute soak will remove it all.
Clean the gas piston, the gas cylinder nut and the inside of the cylinder itself.
For the gas port in the barrel, use a drill of the correct size BY HAND to gently remove any carbon.
I don't know what size the IAI gas port is so you'll have to figure that out.
In USGI Carbines the gas cylinder has a hole in the bottom of the cylinder to allow cleaning the gas port into the barrel.
Do this CAREFULLY, you don't want to damage or enlarge the port.
As an option, you could buy a new USGI gas piston and nut, since these will usually be better quality.
Once you have the gas system clean, DO NOT LUBRICATE IT OR PUT ANYTHING IN THE SYSTEM. Like most gas operated firearms, the Carbine's gas system is intended to be run TOTALLY DRY.
The Carbine gas system is normally self cleaning with GI ammunition.
If you put anything in the system, the super-heated incandescent gas entering the port will burn it into a carbonized, tar-like stick substance that will cause exactly the problem you have now.
This is also why the Carbine should be cleaned with the sights laying on the bench and the gas system up. This prevents lubricants or solvents from running into the system.
To reassemble the system, install the piston and carefully start the nut by hand.
Snug the nut down all the way then tighten down tightly using a GI-type piston nut wrench.
Once the piston is tight, use a center punch with a rounded-off tip to stake the nut in place by moving some of the metal of the gas cylinder into the space between the lugs of the nut.
If you don't stake or otherwise secure the nut, it will come unscrewed and can damage your carbine.
Make sure the punch tip is rounded. The idea is not to put a punch mark on the edge of the gas cylinder, it's to move a little metal into the nut gap to prevent it from unscrewing.
Next, buy a USGI 15 round magazine in good condition, and be prepared to try a different brand of ammo.