The general idea for these advancers is to keep the present slow-speed idle but also allow for more spark advance angle at higher speeds. All of these today have too-soft springs from years of heat-annealing, and we often see these show up as "my bike won't idle back down when hot" problems, despite the carbs being synched correctly and jetted right.
Overall, these bikes in particular were hard-hit in the early days of "emission controls" with "tuning" methods that were less than sound (or even smart) as they were not applied with sound engineering practice as much as with pressure aimed at Japanese bike makers by the US government at the time. The result was for Honda's engineers to simply apply less spark advance angle through the whole RPM range in any bike so affected, as this soothed the agencies adequately to get them to leave Honda alone. In this bike in particular, having less spark advance angle than in previous similar engines (like the Honda Singles of the day) applied enough "change" to get Honda's bikes into the US market. But, the softer springs used in the advancers to still get to full advance by 2500 RPM proved over time to be too soft, resulting in the common "hanging high idle" troubles seen in these pages (with this bike) so often.
One good way to re-tame the idle now is to re-tension the springs, which have become too soft from heat-annealing over the years. I do this by cutting off some of the spring coils to slow the advance down. At least 1/2 turn on both springs is my starting point (same on the 750 engines, too), although more often in the Mid-Four this becomes 1 full turn from both springs. Full advance should not be reached until around 3000-3200 RPM with today's slower-burning gasolines, and this much tension increase gets it close to that.
But, thanks to the aforementioned then-new (and far from perfected) emissions "rules", Honda (and others) hedged their bet against excessive NOx emissions by shortening the spark advance angle, and by a lot as compared to their earlier engines of similar design. This also caused colder-engine running, so Honda (and almost everyone else) went to a hotter sparkplug so it would stay cleaner. If you roadraced back then, one of the first things changed was the sparkplugs (to the CB750's D8ES-L or ND's X24ES-U) along with adding more static spark timing: the bikes would not idle down when hot, but in the pits this wasn't considered a big deal for 'production' racing (where the bikes had to be stock). Today, with our slower-burning gasolines, this becomes an issue as soon as 4 degrees is added to the spark advance at idle.
So, to counter that, we modified the spark advancers (if we didn't get caught) by cutting back or bending back (harder to do) the stopping ears for the weights on the spark advancers. I also notched out the weights themselves a bit where they stop against those ears for another 2 degrees over my competitors (and it worked on long straightaways in top gear). In the end, though we didn't notice it at the time, this removed the so-called "100 MPH wall" that the 500/550 seemed to hit on faster tracks, letting our riders outrun their buddies.
Here's a couple of pix of the one I just modded for the main bike in my last book. Note the thinned-down stopping ears and the notched-in outer edges of the weights: this will bring up about +5 degrees advance over a stock unit, putting it closer to the 750 advancers and the later iterations of this 500/550 chamber design like the CB650 or CB400F, where the required emission controls were moved into the carbs instead of the spark timing. A hint: this really helps the CB550K3 in particular, with the 4 larger-volume pipes, at high RPM.
