[Paulporsche]

Not having the cover or filter probably didn't cause it. I don't know where you are located, but if you're in one of the many areas in this current unusually early cold snap, the cold air might be a part of it. Your cold control pressure may be too high, causing a too lean mixture @ low temps. Resetting your points may have changed your timing slightly, as well. Also, your WUR may be a little out of spec, which wasn't apparent when you put the car away.

You might want to consider a popoff valve in your new airbox. I know it's a "belt and suspenders" solution, but I like the added security for those "just in case" situations.

If you do a search, you'll probably find some info on airbox replacement. There may be something in the "How To" section or Wayne's book.