Outdoor Power Equipment and Small Engines - gas in engine block

Some one out there may have experienced this problem before. Hopefully help will come my way. I have a Sears Craftsman 42" riding mower with a 15.5 hp B&S engine. Recently I started having problems with the engine not turning over. I checked by hand and found it very difficult to rotate the fly wheel. Just by curious nature, I opened the drain plug and discovered that my engine block was full of gasoline, mixed in with the oil. I drained the block and placed new oil in block, filled the gas tank with gas and started the engine. Great now I can mow. As luck would have it, my mower belt broke. Had to replace it, after getting back from the store, the next morning I go out to replace belt and mow. I discovered through my pre checks that the gas level was down and the engine would not turn over. You guessed it, engine block once again had gas and oil in it. What could be the cause of this, do I need a new carburator, I just recently replaced the carb, could this be a bad carb? Hopefully this isn't some type of hole developed in my block. any suggestions will be greatly appreciated.
Thanks all

Your right the problem is the carb. The needle and seat valve is allowing gas to leak by when its not running. Another cause could be a float with a leak in it which would allow gas to leak by the needle and seat.
Some guys have had some luck cleaning the needle and seat with a Q-Tip and carb. cleaner.
If the float is leaking you should be able to remove it and shake it to see if you hear or feel anything in it. If theres nothing in it, its OK.
I would also clean the fuel system. Dirt in the fuel is the problem.

I agree...the needle and seat are not sealing. The float may be sticking due to a burr on the float hanger, it may have gas in the float, you may have trash and/or water in the float bowl, etc...

Remove the float bowl and see what's in it. then wipe the needle tip clean and use a qtip soaked with carb cleaner to swab the seat.