Can anyone tell me the correct Ignition timing BTDC for a 1942 Bsa WM20 ?
I have three books .One has 5/16 second Has 7/16 and the third has 3/8 
email (option): garypitt@talktalk.net
Hi Gary...It's 7/16 BTDC with the ignition set in the fully advanced position on the adv/rtd lever.
If there is anything else you are unsure about regarding the subject take a look on the 'Technical Section' of the website....
I have written a very extensive piece, starting from basic principles, that you will find there...Ian
email (option): ian@wright52.plus.com
Wow! metric ignition timing! We'll be putting litres of petrol in our tanks next instead of gallons!
Well...7/16 BTDC is .437" or , pretty much, 11mm...
15mm is 591"..a difference in the amount of advance of .154" (4mm approx...)
That's a pretty hefty change to the ignition setting...
How will you measure and quantify the improvement, if any?...
Regarding the engine stopping when set to full retard that would be expected and is not necessarily indicative of incorrect timing....
The fully advanced to fully retarded range of the adv/rtd lever is enormous after all and 'retard' settings used when setting for slow running etc. are not that extreme...
Whilst I agree there may be some merit in looking at the ignition settings used with modern fuels (and even they vary) I think it is imperative to be able to accurately assess the effects of any changes made...A 'shoot in the dark and measure with the seat of your pants' method is known to be notoriously imprecise...
Perhaps before and after dyno tests on the same bike would be needed to know exactly what is happening....Ian 
email (option): ian@wright52.plus.com