I have a old stihl 028 wood boss chainsaw that had been running just fine until today--had been cutting tree for aprox 20 minutes & then when I went to turn it off it just kept running! This went on for a good 5 minutes. I tried revving it up--pulled the switch back down to choke & back up to no avail. I did notice sparks whenever I put the switch to the off position so kept it on "run" and ended up pulling the spark plug wire. What would cause this & how do I fix it?

On most chain saws, the on/off switch grounds the magneto. If you are seeing sparks, the ground wire may be disconnected. Have to open the case up to see.

Thanks Ill check it out