There were three different advance units over the run of these bikes. I've not had all of them in my hand at the same time, but could it be that the posts that the springs attach to are in different spots, thus needing different spring lengths/tensions? One of the spring goes onto the moving post while the other end of the spring is attached to a rivet on a movable plate. Could be that the rivet is in different locations on the different style of advance units.
Where have you placed your two different springs? From things I've read that Vech at Bench Mark Works has written, the 11mm spring should be on the side with the question mark stop spring. The 9mm goes on the other side.
'78 R100/7 '69 R69S '52 R25/2
You've perhaps answered most of your query as you formulated the question in your original post.
As you know, the purpose of the advance unit is to advance the timing, with the object being to provide a progressive transition from little advance near idle to max advance when the increased rpms require the most burn time.
The weaker spring will allow an early start in advance, followed by the stronger spring as the outward inertia progressively increases, due to the rpm increase.
Were two weaker springs used, the advance would take place much sooner, or lower, in the rpm range.
Were two stronger springs used, the advance would take place later, or higher, in the rpm range.
Either of those conditions would produce a more abrupt change in the advance curve.
This is apparently what the engineers decided was necessary for the curve transition they deemed optimal.
The article James referenced in the club bulletin is very interesting, both for noting the limits of the roll played by the coil springs, and the broader range of involvement played by the "stop" spring, as well as highlighting the limitations of technology and manufacturing during that era of production.
Along with Mal's great article and the rest of the issue, it's amazing how far the bulletin has come from the two sheets (was it mimeographed?) I'm recalling we got in the seventies.
Many thanks to those putting in all the effort required to bring this product to the rest of us.
That's a good explanation for the earlier design advance unit with two different sized springs. But I understand there were a couple of iterations on the unit and the springs became the same size, 11mm I believe. This suggests that the engineers wanted to delay the advance curve. Why would they have done that...what changed in the engine that warranted a delay in the advance curve?
'78 R100/7 '69 R69S '52 R25/2
You've got me there; I'm only familiar with the 7/11 springs on the Earles fork advance unit, as that's what the half dozen or so advance units here have, so it would be uninformed speculation on my part to suggest if and why changes may have been made at some later point.
It would be interesting to know if the matched 11mm springs you cite provided the same tension as two 11mm springs from the 7/11 pairing, (supporting your theory that the engineers wanted to delay the advance curve) or if they were made of different wire size and/or coil size, and so provided some different spring tension, whether equaling/lowering/raising.
Also be interesting to know if any of the rest of the advance unit design was changed at the same time, (altered flyweight mass?) or remained the same as that with the 7/11 spring combo.
Answers to some of those questions may provide further clues.
Other than that, I'd only speculate (whilst still uninformed..) that probably nothing changed in the engine, other than the design parameters thought necessary for the performance goals of the advance unit.
If I'm remembering correctly, a pair of equal springs were used on the followup /5 model's advance unit design.