My briggs and stratton 15.5 hp ohv murray runs great but when I stop it and wait a few days the cylinder has filled up with gas

I'm not familiar with this one but it sounds like the carb is dripping down into the cylinder. Simply solution would be to shut the fuel off when you're not using it. There must be some sort of fuel line shut-off between the tank and the carb. That would solve the problem that you are experiencing but that wouldn't fix whatever is causing it.

I agree with odinn about the carb dripping. Here are 2 more steps to take. The gas goes past the rings so there is likely oil/gas contamination in the crank. Flush and change the oil after the fix. The true fix is to repair.clean or replace your float and needel valve. I can diagnois the specific issue without further inspection but the needle valve is not closing 100%, could be debris or a bad float.