Hi,
Not sure if you have resolved your problems, if so ignore the following though its an interesting tale.
Just like you, I have been wrestling with similar problems on my 52 Star Twin - running on one and spitting back. In the end the spitting back proved to be primarily caused by tappets that had been allowed to close up. Resetting them initially seemed to cure the problem. Then the LH inlet valve stuck open and bent a pushrod, almost certainly caused by the carbon build up on the permanently open inlet valve. Naively I though the problems would go away after I stripped the head and did all the valves. Indeed the spitting back was cured but the ticking over on one cylinder and the hiccupping at lows revs remained causing a few hairy moments.
Interestingly, the bike would tick over perfectly as a 250 single on either cylinder so proving which was at fault was difficult. As the carb had already been well sorted by the previous owner, I took the plunge and had the mag serviced. What a good decision that turned out to be. There were numerous issues which were sorted out for a very reasonable cost:
1. The slip ring was badly worn and was probably the major culprit, under test it sparked like a catherine wheel.
2. The pickup brushes were wrong and may have contributed to 1.
3. The oil seal had hardened and was beginning to let oil into the armature
4 The cam ring housing was badly worn and had to be shimmed.
The last point was interesting. My mag had been converted to auto advance and the cam ring had of course been locked by grub screws. The wear had allowed the grub screws to push the cam ring off centre so the points gap differed on each side. I knew there was a problem as I could never get the timing set properly and had to equalize with 8.5mm on one side and 11mm on the other but had assumed a worn cam ring and did not notice the points gap changing. I am not sure how such wear would affect a manual a/r situation but it cannot help and shimming is probably not an option.
Fitted the new mag, timing now identical on both sides and first kick start. No more ticking over on one, no hiccuping and no spitting back. Totally different bike. Previous owner took it for a ride today and now wishes he had not sold it. Minor moment of panic when it went on reserve on the way home and stalled at a roundabout just to shake my over confidence.
If you still have problems, maybe some of the above helps. I used the guy down at Langton Matravers who advertises in the VMCC mag. He came well recommended and I can see why. Thorough checkover, only replaced that which needed replacing and back in a week looking like new. I hasten to add that I have no association with the business, just a satisfied customer.
Reminds me of the old story that 90% of carb problems are electrical. For the relatively small outlay compared to the value of the bike I would think a mag checkup by a specialist a good insurance.
regards
Peter