Mystery solved? This is what happens when you don't run a fuel filter, especially on an old car. One year ago I rebuilt the carburetor. After 3 months the fuel filter came loose, I disconnected it, and never put it back on. Time goes on, you forget... well, I am surprised the damn thing ran at all! This is motivating me to drop the gas tank and clean the sucker out. And of course run a fuel filter again. Eeeeesh.