Briggs Stratton mower engines won't start

I have 3 mowers that won't start. They have been sitting for a long time and I think the gas is "flat". All engines start if a small amount of fresh gas is poured into carb throat. How can I get the gunk out of the carbs?

Take them apart and clean them. There are videos on YouTube. I lay a towel or rag on the bench then carefully take them apart cleaning wit spray carb cleaner as I go and lay it on the towel. Them re-assemble.
You'll also want to look in the tank and at the gas lines for rot.