I tend to agree with Stumper,something is amiss here.I am not familiar with the big 166,kind of like to have one though.My advice,for what it's worth.Make sure the coil laminations are getting good ground[clean the area where it mounts to the engine] make sure the kill wire,is not grounded.The air gap,on electronic ignitions,is critical,I would assume it would be about 8 to 12 thou.Make sure the plug is grounded real good,if you try it out of the saw,because it's real easy to cook the coil,otherwise.As has been stated,check for a sheared flywheel key.Good luck,I am out of ideas.