sorry to say but i have seen a few like this and even owned one like it too,look on the good side though you know what it is now ,the primary ignition coils always give trouble on them they do give a weak spark like you say and only if the engines revs are fast enough.just find a good second hand one or buy a new set of primary coils for it and i bet its ok.p.s the primary coils are inside the left hand engine cover.
adam bike doctor (of milton keynes motorcycle centre)
January 2011