I haven't seen this particular topic discussed before so, since i have been working on my tank lately and realized a few things, i thought i would bring it up.
These gravity-feed carburetors use a float and needle to shut off flow once the floatbowl is full. The needle cannot withstand even one PSI of pressure. If there is _any_ positive pressure in your fuel tank, it will force fuel past the needle, resulting in a float level that is too high. It may also cause leakage and fuel overflow when the engine is stopped until the pressure is equalized.
How does a tank become pressurized? There is supposed to be a vent hole in the cap. Simple.. poke a piece of wire into the holes to clear them? NOT! :-)
I happen to have a tank that had been Kreemed by the previous owner. The cap had a little Kreem on it so i soaked it in acetone. The Kreem dissolved and the cap was thoroughly flushed out and i thought i had it licked, but not so..
I took this cap apart even though it looked clean. This cap had two shells pressed together and another small chamber within... totally filled with rusty flakes and particles.. i mean it's volume was full of rust.
So, not only do tanks get rusty, but the caps' interiors also get rusty. If the cap's vent clogs and the sun is out, you will get pressure in your tank. (to test this theory, leave a gas can in the sun for a few minutes and open the cap and hear the vapors escape)
Whatever the cause of pressure in the gas tank, the result is too much fuel may be forced past the needle valve and into the float bowl.