This happened to my dad about 30 years ago, A gas station accidently had diesel mixed with there gasoline. He drove it 7 miles home, the car would not start. I found that all the valves sticking open. Let it sit over night they would close. One revolution of the crank the next day they would hang open again. I pulled the head, (inline 6 cyl). Took it to the machine shop, they knew right away what the problem was and told us about the mix up at the gas station. I would check valve stems for build up of crude. I know it not a lawn mower, just thought I would pass a little info along.