Spitting and spluttering could be whatever you've done to weaken the mixture, or an air leak (- or could be spark related of course). But if it didn't splutter and spit before you played - best look at the last thing you did, first.
Ref the loss of spark, usually when a mag runs fine with the lever retarded but cuts out as you advance it, it's because the internal timing is over-advanced - ie the points start to open before the magnetic flip points of the armature. This is all to do with the relative position of the contact breaker and the lobes on the camring.
(An error in setting the basic ignition timing won't be the cause, so no virtue in spending time endlessly resetting it if you're happy it was done right in the first place!)
Some pointers:
CB ASSEMBLY
Could the cb unit not be properly located on its integral key in the end of the armature? Or is the key so squashed and messed up it's not holding the cb backplate properly where it needs to be? This is the first thing to check. There's only two correct positions for the cb, and only one of them is keyed (t'other is 180° opposite). If it's OK . . .
Are the points themselves OK - heel undamaged, nothing funny going on? If they're fubarred, replacement is needed.
If the points gaps are large, reduce them to 10-12 thou - reducing retards, enlarging advances - and see if it's any better. Just a few thou there can make the difference, although things are set a bit too fine if normal wear over time could conceivably cause the mag to cut out like that.
If still the same problem after playing with the cb assembly you're going to have to dig deeper.
CAMRING
Could the camring be out of position? Unlikely if the AR cable moves correctly through its range . . . but not impossible. It should sit fully in the housing, with its large slot engaged with the stop pin at 6 o'clock and the AR plunger nicely sat in its V shaped groove. If all looks good, then . . .
Could the camring need 'fine tuning' to get it back into range (that stop pin at 6 o'clock is an eccentric, and allows several degrees movement each way of the ring in the housing)? To access the head of the adjuster screw, lift the little pip off it with a sharp blade, and then adjust with a screwdriver. There might be enough wiggle there to get things right again, especially if the pin is currently allowing max advance. If there is no further adjustment available in the necessary direction . . . see 'Below';
HT BRUSHES
Check also that when the mag fires (points starting to open) the HT brushes are on the brass strip of the slipring (which is visible if you take off the HT pick-ups and turn the mag until it shows up at alternate sides). If, at the moment the points open with the mag in the advanced position, the brushes are not on the brass, no sparks - and thinking cap on as one of the above is the issue.
CORRECT SETTING
If you have the mag drive off and can turn it independently of the engine, the points should just start to open a fraction after the 'flip points' of the armature in the fully advanced position. Too early, sparks will only appear with retard applied. Too late, there'll be sparks but they'll get weaker as you retard (they do that anyway) and in extreme cases the retarded spark won't start the thing.
'Below': if the camring can't be positioned to provide sparks across the full range of the lever using the available range of adjustment, despite everything else being 'right' . . .
. . . This is where you may need to turn the camring through 180°. As they're symmetrical, quite OK to do it. But, if you do, then use a 1/4BSF grub screw through the stop pin hole to trial set the camring where you find it gives best sparks fully advanced - ie points opening max 10° after the flip points.
Mark the ring.
Cut a new notch with a small grinding wheel, being sure to cut it AWAY from the position of maximum advance you have established (or you'll create a repeat of the same problem).
Then cut a V notch for the plunger for the cable. Carefully, to allow full movement of plunger without it disengaging at full movement either way.
Then refit.
