Any ideas?

Break in the Spark plug wire at the coil. Replace coil.

Hi,
I agree that it could be the cause!
Suggest that you isolate the engine by disconnecting it from the rest of the machine to make sure it is there. Unplug the ground to the armature either at the armature(if blower housing is off) or externally at the front of the engine or at the throttle control bracket. Usually a white or yellow wire.
Peace,
Clarke