The needle/seat assembly in the carburetor is leaking .

Fuel floods into the cylinder and seeps past the piston(s) and into the crankcase, draining the tank.

Carb needs to be repaired and someone will suggest installing an inline fuel shutoff.

The problem typically happens when a tiny piece of "crud" gets flushed down the fuel line.
It's too big to get past the needle/seat, but lodges in there and prevents the needle from sealing off the fuel flow. (similar to a toilet tank)

Often happens when changing a fuel filter. A piece of crud "flakes off" and if you don't purge the line......

he on the money

Yep, fix it immediately, unless you like the sound of fireworks, as that is what you will hear--a big "BANG", and then no sounds from the engine, except for a clunking, clanging sound, and no work done ever again by that engine. You will probably be able to look intto the block and see the damage, with no disassembly required! BT/DT/RJ
I picked a brand new chipper out of the garbage one morning, that had a block inspection hole in it. And, some connecting rod pieces fell out of it.
Upon further inspection, i found no traces of oil ever having been in it!
I surmised that the guy bought it brand new, brought it home, and in his fervor to try it out, installed the gasoline, but no oil! BANG!
A hardware store owner gave me a new power mower that he had sold to a man, who promptly brought it back, complaining that the oil was "watery". The store man couldn't resell it, so i got it. Took it home, installed new oil and fuel, and it started and ran well. I sold it, and kept track of it. It was looking rather ratty after 10 years of use, but i resold it for $25, just last year.
RJ