If you look at the tank by itself, with the carb off, and the tank in your hands, you will see a hole, and a cup, where the carb
bolts on. These older tanks hold a lot of moisture, which sits at the bottom of the tank, as well as the cup. The cup on these metal tanks replaces the "bowl" on the newer gravity fed carbs, and after years of moisture, the cup develops a lot of rust holes,
which are under that layer of rust in the bottom of the cup, so you don't realize that they are there. I have sanded out these tank/cups, and have placed a layer of jb weld in the bottom , and have gotten several years more service out of these tank/carbs.....