Sounds like the problem my grandfather had every spring.
Might not be the same... but.... He use to winterize his lawn equipment himself. Used StaBil. He said they told him it was the best stuff. Well, his train of thought was, IF a little Sta-bil was good, twice as much would be better. Three times as much etc etc....
Well..... come spring I would get a call to help him get the motor's running. They fired a little then stopped. Turns out, too much of a good thing can be bad. When the stuff burned, it created a conductive coating on the spark-plug. Plug looked ok, nice & tan, but no spark. So, how did I find this out the first year, similar method, holding the plug against the block, but.... then I OOPS and pulled away a little but and still had a little bit of skin on the engine cover and was too close to the metal of the plug, while pulling the starter cord
Boy did I jump...... Now I just take new plug over every year with a freash can of gas. Old gas gets mixed into my tuck and his car.
So my suggestion , check the gas, then run and get a new plug. Nothing fancy, just a standard plug, they run 3 or 4 bucks.