This is your answer right here just staring you in the face. Why don't you just suck the gas out of the tank and put it in your lawnmower and get 5 gallon of fresh high test gas. That gas had to have water in it or something was wrong with it. The gas was probably changed to winter gas and maybe the formula is not correct. I have heard that a lot of people are having trouble with their lawnmowers and other power equipment because of the ethenol put into the gas. I am getting this from the small lawn mower shop in my area. The only other way to have this problem on both cars at the same time is if it started to rain a lot during this time then I would say it is ignition related because of the excess water in the air. Then you should put in all new wires and plugs and caps and rotors. If you buy junk wires you will get junk results.
