I agree re with John re the spray - it should run without fuel - but didn't you originally say it was running for a minute or so, then 10 seconds for each start after that?
What has changed? Have you changed any ignition component settings or is there a reason to think the ignition timing has changed?
If not, I think you are still at the carby - or at least, I wouldn't change anything elsewhere until I was sure the carby wasn't the problem.
Having said that, sometimes a weak spark can completely disappear under compression, when everything else seems ok. Can you borrow another battery? If every thing else the same and the symptoms are getting worse, the battery could be failing. If it's not sealed, you might be able to get hold of a hygrometer or have it load tested.
'51,'56 Squares, '48 VH, '27 Model C, R67/2, Mk IV Le Mans, '06 Super Duke and Ariel projects.