[Mike Taylor 3601]

Try removing timing dial control I've seen starter savers,retard boxes,the start retard that is built some boxes. {6AL doesn't have}etc. go bad and cause start issues. Try car with just 6AL no extra boxes and see if that fixes it if doesn't try to borrow another 6AL from someone to verify your's is OK.Make sure starter is grounded to block scrape paint from block where starter mounts could be weak ground and just the slight added load of ignition could make the difference.As Joe mentioned check timing while cranking
could help diagnose.
Mike Taylor3601