Ah the mystery of the murdered spark plugs. Once upon a time in a dark and distant past, a bike rider would place a new spark plug into a bike and ride for months, if not years. Sometimes the plug would oil up when the rings were sorry and foul. As technology advances and fuel improve, spark plugs seem to last less and less. Ready to foul at the slightest excuse. Some say dodgey electrics. Some say dodgey fuel. I have about 6 2 strokes running between the family. Some old and some new. All of them have fouled plugs and are keen to at any given time. Has a near new YZ125 foul and plug and when I went to get a new one on my WR2502t, it fouled one.
The modern fuel sucks as much as the new spark plugs. You remember when you used to leave some fuel in a container it would all evaporate in a couple of hours. Leave the same amount of ULP now and your are left with an oily residue.
Only use the freshest fuel.Use the left over in the mower. Empty what you don't use Keep a good supply of spark plugs. I use sythetic oil at 40ish to 1. Mobil premium fuel i find the best, Caltex then BP. No others. Not Optimax from shell. Have bee told it is frm the devil.