It happened to me one time. I had converted a Homelite and it sat around the shop about 3 years before I used it. Hits weren't real bad but I was getting them. I didn't look at the plug because I had installed a resistor one when I converted the engine.

A friend come over and suggested about the plug and sure enough, it wasn't a resistor plug. I felt like a big dummy.

One of my sons must have swapped it out to use in a weed whacker or chainsaw?

Enjoy,

Jim