Alex,
A common fault with Lucas magdynos is that the paper cage that surrounds (and insulates) the bearings sometimes disintegrates, allowing the armature to shift sideways, thus giving wildly varying points gaps. This usually results in the symptoms you are experiencing. Try gripping the points centre bolt and check for any movement either axially or radially - if any movement is detected, the bearing cage has probably disintegrated, in which case the magdyno should be removed - quite a simple task as on Mark series machines the timing gear comes through the hole in the back of the timing chest, and so does not have to be disturbed. Working on the points end of the mag is much simpler with the unit removed from the engine. Firstly, remove the centre bolt that retains the points assy - then lightly lever the points assy until it frees from the taper. remove the 4 screws that retain the back plate. This should come away complete with a couple of shims and the outer track of the tail bearing. The loose outer track can be rectified by cutting out a new cage from a suitable thickness card. Improved support for the bearing can be gained by 'Aralditing' the bearing and cage into the backplate. Do not try to make a metal sleeve as this will allow the armature to 'earth' though the bearings - causing early bearing failure. When repaired, the magdyno will of course have to be timed when refitted.
Good luck with the repairs,
Eddie.
