Critically, the ATD needs to return fully when at rest so that upon startup you can be sure the bike isn't going to kick back. Once the bike is started it is important that the bob weights open fully to achieve full advance. What happens between startup and full advance isn't as important as some might suggest. If the springs are strong enough to return the ATD to its closed position they are probably about the correct strength. I have found that very light springs won't overcome the friction in the ATD itself and, having fiddled with ATD's for years I've got a pretty good grasp of which springs feel about right. Remember we are talking about low powered bikes which are quite forgiving in terms of their setup. The ATD needs to operate smoothly so that there aren't any sudden 'notches' in its operation and needs to achieve full expansion somewhere approaching 2000 r.p.m. (I guess) or the pipes will turn blue due to retarded ignition. The springs I use are cheap and cheerful, they feel about the same strength as originals, so I just make sure the ATD returns fully by 'adjusting' the springs so there is a little bit of tension applied when at rest.
I get occasional phone calls from customers saying that the springs aren't strong enough because the ATD works in the hand but not on the bike when at rest. This is normal due to the various counter-forces in the magneto. If you turn the engine over very slowly with the kickstart you should see the ATD close completely which reassures you that the bike won't kick you back next time you start it.
