Oy vey.
There is a dob of solder on the end of a float as well as on a side of all carb floats. The hole under that solder is the last thing closed up when the float is built.
When you repair a float, the first thing you do is remove the solder from that hole. You can then shake most of the fuel out. Let the float sit in the Sun or next to an incandescent lamp for an hour or so to boil all the fuel out.
You then solder up the seam where the leak is and allow the float to fully cool.
Solder melts at about 200 * and quickly heats the air inside the float.
If you haven't opened the breather hole, the heated air expands and tries to force its way out through the molten solder in the seam you are trying to close.
When you are through with the seam and the float has completely cooled, use a solder gun and a small drop of solder, you can quickly close the hole up before the air inside has a chance to heat up,
