I have a GI M1 Carbine that seems to short-stroke about twice in 50 rounds.  Ejects the brass but doesn't load a new round.  I tried NIW 15-round magazines but no luck.  I always use factory-loaded ammo.  It gets cleaned, oiled and grease where appropriate after every use.  Any ideas for a quick fix?

There's a LONG list of problems that can cause short-stroking in the Carbine, but other than bad magazines or trying a different brand of ammo, the Number One cause is a fouled gas assembly.

If the gas assembly gets carboned up, or if, over time, oil and bore solvent leaks down into the gas cylinder and carbonizes, the gas piston can get sluggish and freeze.

A quick check is to field strip the carbine and inspect the gas piston.  It should move in and out of the gas cylinder freely, and should have about 3/8" to 1/4" movement.

If the piston seems to be Ok, there is a chance the gas port from the barrel is clogged.

The fix is to buy a GI-type gas piston nut wrench and disassemble the system.
To do this WITHOUT damaging the delicate assembly, get a GI manual that shows the proper method.

After the system is disassembled, clean the gas cylinder out with a brass scraper, brushes, and solvent.
Clean the gas piston nut.
Clean the gas piston with a brass scraper, or better still, with a Lead-Away type cloth.  

DO NOT use any steel objects or abrasives that can scratch any part, and be careful of the threads on the gas cylinder and nut.  If scratched or marred, the parts are ruined, and the Carbine will foul very quickly.

The gas port can be cleaned by using a piece of brass wire, or if you're VERY careful, the appropriate sized drill bit, turned carefully BY HAND.

Once everything is cleaned of carbon, reassemble per the manual, and re-stake the nut, again per the manual.

NEVER oil the gas assembly, it's designed to run DRY, and clean the Carbine upside-down to prevent oil or solvent from running into the cylinder.

GI Carbine manuals are available online here:

http://www.jouster.com/lanestips/

http://www.surplusrifle.com/downloads.asp

http://www.surplusrifle.com/downloads.asp

Thanks, that's some good info and links.  I didn't know about that gas cylinder, so I removed the carbon I could get to.  It moves freely now.  I'm going to try another box of ammo and if that doesn't work, pick up a wrench and try getting the inside.

If I may, I would also like to suggest you swap out all the springs with new ones from Wolff. They have a complete kit for around $12-14 that includes all the springs.

If you have an original recoil spring you might be beating the rifle to a slow death. A new spring is awfully cheap insurance, IMO.