Ignition advance
I took the Rudge out yesterday, having restored the cr to about 7.1:1. It was about 6.8 before. While it is timed by piston drop, I know it's about 40 to 45 fully advanced. It has a manual retard. What was interesting, and what bears out points made below, is that while it ran easily at full advance at 80 mph, when accelerating from about 50 to 80 it was clearly happier with quite a lot of retard. Once a speed had been set the ignition could be fully advanced and it was happy. Putting numbers to it is difficult, but I'd guess from the lever movement that it accelerated better retarded 15 to 20 degrees. Presumably the more sophisticated car systems (of the 1970's - I've no idea how they work now) with a vacuum input in addition to the centrifugal input would cope better delaying the advance while the throttle was wide open.
What I've also read is that while a Lucas ATD in good condition will steadily advance the ignition over the first few thousand revs, what is likely to happen on older units is that it stays retarded while the revs build, then lets go. They're sticky, in other words. I believe this is what prompted Francois Grosset to design his distributor, since if an ATD sticks fully advances, a starter motor transmission will have a life nasty, brutal and short.
All of that said, my feel is that you're getting full advance too early, either before the design speed, or before the speed it should have been designed for, and if the ATD is in good nick, moderating the right wrist is the answer. One famous Manx Norton (fixed 34 degrees advance, 11:1 cr) tuner insisted that his bikes were raced with a slow-action twistgrip - infuriating his riders - because "they meter the fuel better". Hmmm.....