from what been said it sounds like you are sitting on a granades bit like the old rotax 532 !
I think I be looking at replacing a jab with a 912 s or a 582:amazon: I feel safer
.I have a friend who but a jab in a lightwing and he always got the heads of the thing he always trying to get it right ,,
but in saying that I know of others early jabs that are going fine and still getting around everywhere with no problems so to me it sound like a late model issue '