The prewar manuals list 7/32" BTDC for ignition on full advance. Then the WD manual, for the same engine, lists 7/16" BTDC. The 7/32 must be a mis-type.
Anyway last weekend I did some tests. First: set at TDC of full retard. I did not run it; but on full advance the setting would have been perhaps an inch or more. So the lever was pulled back maybe not more than half way to obtain 7/16" - so there wasn't much retard left.
So then I set it so the 7/16" is reached with another 1/4" or so on the circle on the lever top. Marked it with a sharp paint line.
Now to test. Yes: 7/16" is fine. But I was hoping for maybe some further benefit with a touch more advance since I have had the feeling it is a bit retarded. And I think it does benefit a little bit but not very much.
Other posts here suggest 1/2" and I think that's where I shall set it if I bother to alter it again. At present it does not allow me to retard it enough to get the ultra-slow tickover I could get when 7/16" was as far as the lever would pull it.
So: next weekend it's going to go to 1/2" with maybe another replacement paint line on 7/16" just really for interest.
But it's clearly not all that sensitive (as we mostly already know) - but if it is retarded back to 7/32" it turns into a slug! I wonder how many owners put up with the 7/32" and don't know what they are missing?