The dangerous part is when you have big vacuum advance, but it doesn't go off as quickly as you would like - result: destructive knock (even for a second or two).
Rather than tune it with an adjustable can (Crane etc.): a small air solenoid (I use Bimba, it's 3/4" OD, 1-7/8" long and cheap) in the hose right at the can shuts down in .1-.2 seconds when the micro-switch on your throttle linkage hits, or you can use a button on your shifter.
Yes, there will always be a delay while the plate relaxes to normal position, not sure if a stronger spring is OK.
For sustained light throttle cruising, 60 years ago J.C. Whitney, Warshawsky, Honest Charley, etc. used to sell a thumb wheel with a cable attached. Device goes on your dash, cable moves the advance plate through the original can's linkage hole. To use: watch your vacuum gauge and move it to get the highest reading. Depending on your cam and gearing, higher speed may need more or less advance. Yes, for those WOT bursts you have to invent a quick disengage so you don't have to thumb it 10 times.
If you like to fab, a mechanical solenoid can pull and release a similar cable from Position A (relaxed) to Position B (+20 degrees) instantly with a strong return spring. If it's on a bell-crank, different linkage holes allow choosing among several advance levels.
Cables: quality products only, lube well, only large bends, adjustable for slack. Your bike shop has some choices.
i took a que from the factory distributors and use a primary/secondary mech advance to stage the mech timing. the factory vacuum cans are adjustable and that's a big help. i also limit the amount of vacuum advance in the units to try to keep total for mech and vacuum to about 50degrees. i adjust the vacuum opening to around 10" of vacuum so anything under 10" diminishes the vacuum advance quickly. a fly in the ointment here is there are 3 different types of springs used in the canisters. i much prefer the 9 degree factory units over any aftermarket and they use the medium spring. so far this works for what i have but i think the key to all this is how much vacuum the engine makes. my experience has been the higher vacuum (stock type engines) are more temperamental than lower vacuum performance engines.
