Real long shot, but do you have another coil hanging about ?.... I have heard of a weak coil causing plugs to spark in the open air but not under cylinder conditions ..... exceeding rare but then so is an engine with everything working defying a decent mechanic for three days.
K
Mark's already tried a different coil and ballast. Very frustrating!
Just to update, it does have a ballast resistor but I have even tried another ballast resistor and coil - nothing changes
