Howard now seems to have found the cause of his backfires to be mechanical but if you do have coil ignition starting problems it is worth trying to check voltage under working conditions with just the battery supply, especially if you have an electric start. I had a problem on an electric start bike (not a Vincent) and the voltage at the coils was OK static but during cranking was low. Better earth connections to the coils improved things. Some electronic systems can also default to no retard when the supply voltage drops below a threshold. Early Boyer units suffered from this. I can't comment on newer units as I have no experience with them or other makes.I would not let the battery off so lightly voltage is not everything I have just fitted a Pazon to my just built Trigrumph unit 500 the battery was 12.6v but she did not run clean put on a new battery same voltage and she runs smooth
Somebody said once it's amps that do the work.