Getting additional advance from a stock advance unit

[DewCatTea-Bob]

By: wcory...
" You could just bend them out a bit, being mindful that at one point they will contact the timing cover. "

____ Back in the '70s I played-around with both types of AAUs, and I bent-back the stop-tabs on one of them (which was no easy job), with an intention of then drilling small holes through the tabs to insert new stops into, but, I didn't get the tabs flattened-out well enough and so decided I'd instead braise-on some added metal to then carefully file-down so as to make nice dependable weight-stops.
Unfortunately I never got-around to getting the braising-work done.
Also, I don't recall ever being concerned about the extended-tabs possibly reaching contact with the timing-cover's cavity-wall. _ Are you certain that that possibility is an actual-contention (with just the material of the bent tabs themselves) ? _ I'd tend to doubt that possibility because I don't recall that ever being any concern.


" Is it correct to assume though that with a different ignition setup it would go back to the zero setting? "

____ Of course, if the original advancer-range span is altered, that means the static-advance set-point would then need to be re-adjusted so as to end-up with at-least 27-degrees BTDC max.advance.
____ The reason the 450R/T has different ign.timing from all other 450s is due to the fact that it's ignition-system, (borrowed from the early w-c 250/350 SCR-models & much like that of the similar n-c 250s), is powered by a single DC-pulse (from the 4-pole AC-alt), and since that power-pulse lasts just 90-degrees of a crank.rotation and only offers near it's full-power for about just 1/4th of it's lifespan,, the AAU's range-span must try to keep the ign.timing within that limited range. _ Otherwise the ign.spark may be too weak during kick-starting.
__ And that's the only reason for why Ducati chose to use the 18-AAU instead of the 28-AAU in that particular 450-model.
I have however had success using the 28-AAU in such models which retained their stock 'energy-transfer' -(not actually "magneto",[like everyone thinks is cool to say]), ign.system,, but then more care has to be taken to make sure that the needed power-pulse is timed so as to be producing at least sufficient power at both the static-setting & full-advance point, (which can be somewhat complicated but not too difficult to accomplish).


Dukaddy-DUKEs,
-Bob

[wcorey]

Is there concern that the locating pins in the forks are about to slip clear if we go any further out towards the stop tabs?

Not really, unless you ground an excessive amount off the outside of the weights, both where they hit the stops and near the outer ends where at that point they would have otherwise been limited by contact with the walls of the recess it sits in (within the timing side cover). There is only a problem If the mid/widest point on the pin goes out past the end of the fork, the cam can 'flop' either way, randomly producing more or less advance.
Getting still more 'out there' on mod's, you could make new pins with a non-linear radius that would allow you to let the weights out a bit more while avoiding the 'flop'.


Bill

[Bevel bob]

It would be most illuminating if we could compare modified and standard AAU's using a strobe to see what is actually happening, I suspect that at tickover the advance is jomping around (not helped by a singles power pulses).I think that modifying the retarded position would weaken the spring loading and make this worse.39 degrees advance for max power? ,seems a lot, to achieve this would mean 11degrees static ---too much on my mtor for reliable kick starting.Why do we need 39?? my Norton uses 28 , It has a better combustion chamber,better swirl and long stroke.Perhaps we can reduce the advance on the 250 if we improve the squish and fit twin plugs, I understand that there is not much max power gain on the 250 from twin plugs ,but what is the reduction in timing? must give better running and reduced stress ?.

[DewCatTea-Bob]

" I think that modifying the retarded position would weaken the spring loading and make this worse. "

____ Yes it's true that the spring-loading will then be slackened and the fly-weights will thus consequently begin working in opposition to the resistance of the points-cam follower-heel somewhat sooner in the RPM-range, but unless the springs are totally limp, there should still be no auto.advancing taking place during kick-starting (where it matters), until after engine-revs take-over the cranking-speed.
And, although the advance-curve may then start-out at a slightly steeper rate, that's only going to help the engine get up to power all the sooner. - (Not that it'd be any notable difference however!)


" 39 degrees advance for max power? ,seems a lot, to achieve this would mean 11degrees static
Why do we need 39??

____ That's for the 250s...
You should've already been aware Bob, that the fastest 250, the Mk3, has a maximum static ign.timing of 23-degrees BTDC, with max.advance of 41-degrees BTDC,, so "39" is a fair compromise !
The only notable difference between the Mk3 & M1 motors is their ign.systems, so that must be responsible for the (minimum of) 4-MPH difference in top-speed.


" Perhaps we can reduce the advance on the 250 if we improve the squish and fit twin plugs,
,but what is the reduction in timing? "

____ Dual-ignition on a 250 helps fuel-milage and allows for cooler cyl.head temp, (both minimally so),, due to slightly retarded ign.timing required, such as 35 to 37 (instead of 39 to 41) BTDC.


Dukaddy-DUKEs,
-Bob

[wcorey]

Also, I don't recall ever being concerned about the extended-tabs possibly reaching contact with the timing-cover's cavity-wall. _ Are you certain that that possibility is an actual-contention (with just the material of the bent tabs themselves) ? _ I'd tend to doubt that possibility because I don't recall that ever being any concern.

I dunno, I don't see a lot o' room in there, maybe there's some variance in the cavity sizes from one to another, it's only clearance after all...

[DewCatTea-Bob]

" I dunno, I don't see a lot o' room in there, maybe there's some variance in the cavity sizes from one to another, "

____ Thanks for the close-up picture Bill ! _ Although I would've taken your word for it (after you had checked to make sure about it yourself).
It's been too long since I last had a look inside that set-up ! _ And maybe I never got around to actually trying to insert my (AAU which I had bent the tabs on), or, maybe I had indeed TRIED and that's perhaps the reason why I never got to getting the tabs welded-up (for new-stops) onto.
__ I do think that getting more advance-range is important enough to go to the added trouble of running a grinder-bit down-inside there to create a little deeper recess-slot into the cavity-wall, for the purpose.
__ So still wonder what you think about my old idea of flattening-out those tabs & adding new stops... Would it work well enough (to get a few added degrees), or would there be still other consequences ?


Fun-Cheers,
-Bob

[wcorey]

__ So still wonder what you think about my old idea of flattening-out those tabs & adding new stops... Would it work well enough (to get a few added degrees), or would there be still other consequences ?

One consequence still goes back to the ' point of diminishing returns' situation, I'll try to explain in more detail than in the previous post...

Look at the weight on the left in the picture and envision the radius that the cam opening pin will travel in relation to the weight pivot pin, then look at the cam fork and where it sits on that opening pin.
At this point the opening pin is traveling more away and out of the fork than it's pushing it sideways while at the same time the midpoint/widest part of that pin is about to go past the ends of the fork. The combination of these two situations can let the cam then start to 'flop' back and forth but more likely back, since the direction of rotation and drag from the points opening heel would tend to pull it that way.

Assuming the geometry of your particular unit would make it adventitious to let the weights go out a bit further, then it would likely be just as effective and much easier to simply bend the tabs out and possibly also make them slightly thinner on the outside to gain more clearance. It doesn't seem like there's enough room in either of mine to go out far enough to justify a 'complete transplant'.
Grinding a small area of the weight where it contacts the tab is obviously the other option and I really think the loss of inertia from the small removed mass would be minimal and you could still bend the spring tabs in to compensate. Could compromise and do a combination of all.
Bear in mind that it's all case hardened (which is why I always say 'grind' rather than 'file') and heating it to braze on a tab on or even just taking off surface material may allow for unwanted wear.

[DewCatTea-Bob]

____ Thanks for your follow-up Bill, (but I didn't mean to have you be bothered with such an extensive reply).
Hopefully Mike & I are not the only ones here who appreciate such fine detailed writings !


" heating it to braze on a tab on or even just taking off surface material may allow for unwanted wear. "

____ I thought of adding material (to fashion new stops out of), by brazing would be do-able because within quite a number of old DUKEs I've taken-in over the years (in the past), I've found very many of the 18-units with their fly-weights braze-wielded (fully open, & springs missing), to the stop-tabs.
(Since I had found so darn many AAUs that way, I always wondered if the factory had done it or if some cycle-mag had advocated the practice for some hot-rodding reason ??)


Nice-Cheers,
-Bob