A loose nut on the advance/retard lever is probably not helpful as any vibration means that the ignition may retard of its own accord (assuming you have a tight wire advance mag).
I would ignore the choke i.e. make sure it is in the fully open position.
What condition is the carb in? The main reasons these bikes are difficult to start are due to either a worn out mag (not the issue here), a worn out carb or poor timing.
Regards
Pat
ps Once your bike is running, fully advance the ignition and leave it there (unless it pinks a bit going up steep hills). Running any bike retarded is not good for them and particularly bad for side valves.
I cleaned and rebuild the carburettor and to my novice eye it looks clean and functional.
Your point about running the bike fully retarded has been noted; I understand it can result in the exhaust valve becoming too hot, hence causing new problems.
I'll share my own experience here. My WM20 has a rebuilt by the REME engine, unused til I fitted it. Good spark, nice clean carb. It didn't want to fire up, and back-fired through the carb a couple of times. Like your bike it did eventually start and ran somewhat poorly for a while before settling in to a nice steady run.
After some frustration I discovered that I was over-tickling the carb, effectively flooding the engine. Now I simply turn on the fuel, don't touch the tickler, set the advance at about 7/8 advance, no choke needed. It usually fires on the first kick.
I was judging this bike by the standards of my other pre-war rides, where flooding the carb usually permits an easy start. This is not always the case!