The Stihl thread

661 had at least 3 coil updates that i know of, i swapped coils with no ill results on them. Solenoids too, but i never messed with filter. I have heard some of these mtronic saws will not show spark with the plug out and touching jug. Can't wrap my mind around how that works but the guy at dealership was one telling it. I would vote coil though going off the time frame