Last winter my exhaust valve got stuck from time to time, leaving my by the side of the road, without compression.
First i thought there maybe too much coal on the valve stem, but after 2-3 minutes, i could start again and move on.
Also gave a little more valve play, with no result.
It turned out to be the ignition timing, only had 7 mm left in full advance, instead off the required 11 mm.
Obviously it had changed over the years....
Now i have the full advance set in the middle of my combination levers range, allowing me to easely change the timing.
Can't say I agree with you setting full advance in the middle of your levers range. This makes it possible to really over advance your engine causing even more problems.
Stick to the tried and tested method as described in the technical section of this website. At the very least you are creating a problem for the next owner. If your ignition timing has altered over the years there must be a reason maybe your points gap has altered or the points are worn.