Just like Rhoderman says, the nipple will come out and the tank needs to be rinsed very, very well if you intend to apply an open flame. And you may have trouble depending on how it was repaired before. Personally I don't like to use Teflon tape on gas lines, but that's just me.
If you have tightened the nipple and it still leaks then the problem probably isn't with the plumbing... I've never had to repair a tank from a 100 but I've done several of the round style 7hp tanks, the larger square type tanks may just have the flange "crimped" into the hole, but I'm not sure.
On the round style tanks there is a square flange that is soldered into the hole in the bottom of the tank that the nipple screws into, chances are the solder is cracked/deteriorated or whatever. That is most likely where your leak is.
Wire wheel all the paint/rust off the bottom of the tank where the flange is, (once you get it good and clean you should be able to see the outline of the flange in the solder). Screw a longer nipple with a cap on it into the hole, put some water in the tank (you gotta rinse it anyway) stand the tank on it's end on your bench, and watch carefully you will see water droplets starting to form where the leak is. Keep in mind that water won't rush through a leak like gas will so you may need to apply sideways pressure to the end of the nipple to "open" the crack up a little to find the leak. Once you have found the leak, just clean and solder it like you would anything else, but use as little heat as possible so you don't warp the sheet metal or heat the solder so much that the flange falls out of the hole (I had that happen once).
I have soldered several tanks with good results. I know that the thought of doing this is kind of nerve racking, but the THOUGHT of doing it is really worse than actually doing it.
There are also different products on the market made to "stop" fuel tank leaks, but I've never used any of them.
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