First of all IME the B50 is one of the most sensitive bikes to start.
After a couple of pints I would describe it as a real bitch.
I suspect that your battery
is well on the way to going to battery
Quite possibly a bad cell.
With the capacitor disconnected the battery
has only to power one set of points which it probably can do with just 9 or 10 volts.
With the battery
disconnected the capacitor is doing what it is designed to do.
With both battery
and capacitor connected I suspect that the capacitor is discharging into a bad cell and very little electrical energy is left for ignition.
My next step would be to load test the battery
or if you don't have a load tester then try substituting a known good battery
No need to fully instal it--just jump the second battery
while making sure that the first battery
is out of circuit.
BTW---IMHO a battery
load tester is a really good tool to have.
If you are in US you can get a reasonable one for about $35--well worth the money.
Even new batteries
are often total crap these days (made in C----).
A friend of mine with a HD asked for my help as he had a bad battery
and had bought a new one but it made no difference.
"Cant understand it" he said "it is a new battery
so it can't be the battery
I load tested the battery
He took it back to the store and returned with another new one.
I load tested it before he fitted it--again total crap.
He rook it back to the store and got another new one which was OK.
Three new batteries
to get a good one!