I did a carb clean for a good friend on his ExMark walk behind mower with a V-Twin Kawasaki. I put new fuel in it, mowed my lawn with it, worked excellent. Gave it back and a week later he says it's acting up again. I stop by his house, drain the fuel, and the fuel tank (which he just filled) was 1/2 full of water.
If you use E10 gas, the best way to avoid this is to buy fuel in smaller amounts and use Marine fuel stabilizer (Stabil Marine is good). I buy my fuel 2.5 gallons at a time, and mark the date on a piece of tape on the can. If it is older than a few weeks I dump it in the car.
My grandfather is constantly having issues with his fuel, but since it is hard for him to get out, when he does he fills all his gas cans at once, and they sit for a while before he uses them.