Motoscrubs.com

Good morning-

Warm weather is coming- I am planning ahead regarding my 1966 250 scrambler, that has had such difficulty starting, for years (actually, read NOT start, but DOES start, with a DC battery supplying electrons to the "DC re-wired" ignition coil). I have been reading, and re-reading all I could find on this forum, information about the AC alternator/ignition system that is original to this scrambler. Dew-Cat-Tea Bob wrote a LOT , with a LOT of advice about using that original system. He seemed to think, that with the proper timing, it would work just fine. I COULD go for a new electronic system (Lacey Ducati has one), but... it might make more sense to actually figure out WHY my Ducati won't start with the AC system, and just get THAT working so that it will start.

I have spoken about this 6v AC system with a friend, who understands all things involving electrons, and have sent him copies of wiring diagrams, and links to Dew-Cat-Tea-Bob's postings... My friend is intrigued, and while he truly DOES understand electrons, he is also more than willing to learn more. We plan to hook up his scopes to the wires of the Ducati's alternator coils, and then will pull the sidecover and flywheel, and figure out the timing of the AC pulses, and figure out when the points should open and close. Using the AC system "as God and Ducati originally intended".

BUT... I believe that my Automatic Advance Unit is a "fixed" non-advancing system. If I read Dew-Cat-Tea Bob's postings correctly, he does suggest that a swap to an AAU (28 degrees?) will help with starting (saving ankles). And I would guess, with running, as well. After all, why wouldn't you want to have a different ignition timing, for an idling motor, compared to a motor at 3000-4000 rpm or more. I would think that advancing the timing would be helpful/smart. And again, after all... isn't that what a proper electronic ignition system does?

Therefore, I am looking for an automatic advance unit (AAU) to probably swap into my 1966 Ducati narrow case 6v AC system scrambler. I believe that was is IN my scrambler now, is part number 0601.34.280 (also found in a ducati 250 mark 3?) I believe what I WHAT to acquire is part number 0400.34.280 (as found in my 1966 Mach I).

Does anyone have an AAU for a narrow-case. to sell, or perhaps a lead on how I can find one?

Sincerely,
Bob Woolner
Hillsboro, New Hampshire USA

Hi Bob,
I have an aau from my nc 350 Sebring which is now redundant as I have fitted the electrex system from Nigel. Although I have yet to get to the stage to have her running, but can’t see why it won’t (famous last words) .

If you can’t get one in the US, I’m happy to post it out to you.

Regards

Dave

Any good?

https://www.classicracing.nl/product/04 ... ic-advance

Hi Bob

a ac ignition system on a Ducati single uses a AA 367B advance unit . it advances 18 degrees and had a diff breaker cam
to saturate the ac coil .

the dc ignition system uses a AA 359 unit . and that one advances 28 degrees

the units are clearly stamped so you can see what AA unit you have .

Eldert

Good morning all-

Thanks to all who have replied, so far.

Dave, thanks for the offer- as you note, it will be better, if I can find an AAU here in the USA. But if nothing shows up, I would be interested in your surplus AAU. Are there any numbers stamped into it? Assuming I can use it, I would happily pay you and certainly pay for shipping.

Thanks to Nalimugmug for finding that unit in the Netherlands- if nothing in the USA turns up, and if Dave's surplus piece doesn't work out, I would certainly consider that one from classic-racing NL.

And thanks to Eldert for his knowledge of 18 degree vs 28 degree AAU. In my previous readings from Dew-Cat-Tea-Bob, I note that he seemed to think that a 28 degree AAU would be a reasonable interchange, at least on a MK3 Ducati, that he was advising about, in a lengthy post. I admit, I am having trouble finding that post-thread right now, to re-read, but... I thought he thought... the 28degree AAU was appropriate.

Eldert- would the "different breaker-point-cam ramp" (comparing 18degree AAU and 28degree AAU) make that much difference? I don't mind being more particular in my search. And Eldert- again, my goal is to make this mostly-original 1966 Ducati 6v AC system work. I can only imagine that an AAU would be better than a fixed-advance. Yes? 18degee is preferable, according to you... would a 28degree AAU work, but "not quite as well" but better than fixed-advanced?

I think I had best do some more research in the archives here...

Does anyone in the USA have an AAU for a narrow case? Probably an 18degree unit? An AA 367B? (But maybe I'd be happy with a 28degree AAU?)

Thanks all,

Sincerely,

Bob W
Hillsboro, New Hampshire USA

Seems bloody expensive though!

Good evening Dave-

Huh. I had not seen that- when I search on ebay, I have been searching key words Ducati single ignition but those never turned up this aau you found. thanks.

AND... then I found this!

https://www.ebay.co.uk/itm/225372128307 ... media=COPY

So two of them to choose from, both here in the USA! The one I found is a bit more reasonably priced.

Does yours have numbers on it? Assuming it is from a battery-ignition, it is likely the 28 degree AAU, which according to Eldert, is the wrong one for me, and the ones here on ebay are correct...

Sincerely,

Bob

Just to summarise my understanding of the Ducati AC ignition system:

There's no such thing as an AAU with fixed ignition timing. The advance range is always variable, if working as designed.
The alternator rotor is mounted on an unkeyed taper, and its timing is critical in an AC system, not at all with a battery system.
Charts are published in Ducati service manuals, showing the required rotor angle.
With AC, as the available voltage is continuously varying from zero to maximum as the rotor spins, there is a usable range wherein the sparks are possible.
Outside that range (18 degrees) the voltage would be too low. That's why a 28 degree AAU is not appropriate for AC ignition.
28 works best on a DC bike that supplies a steady good voltage. That system doesn't care where the rotor is.
18 is the best compromise for AC, and will likely give the odd kick-back unless the starter lever is given a vigorous swing. More revs = more volts.
There is virtually no advantage to have an AAU on a light motorbike, other than to make starting easier, some would argue.

If you intend to ride in traffic on the roads and perhaps in the dark sometimes and want a bike to kick start with some sort of reliability then a conventional DC system with a better alternator and suitable battery , dc coil, points cam and AAU that provides the static spark at around 5 degrees before TDC will make life much simpler. The Kickstart mechanism is well known for its fraility and kickbacks will soon have you looking for those very scarce parts. The awkward LH kickstart will tempt you to start the bike on the stand and for that it needs to be an easy starter or the stand will fold up pronto. I have an orriginal 1966 (UK Mach1) 60 watt alternator modified to work with a modern rectifier at 12v and works well with an electronic points assist unit. Often starts first kick . A better alternator is key . Too many of these bikes were dumped because of bad starting and lack of understanding of the AC ignition system which was only really barely suitable for competition use.