Recharging 250 MK1/MK3 Magneto

[DewCatTea-Bob]

to be reededited

[amartina75]

I am bumping this thread to the top because I have daone a fair bit of searching and this seems to be the best thread I have found related to what I am currently dealing with. I have not had a chance to read all of the postings to this thread yet, and some of it is over my head. I have a 66 scrambler that I am working on, with the help of my brother. It does not have the original engine in it. The original engine i do have is #101291, the engine in the bike is #86687. I have done a fair bit of work to the bike already and it is nearly done, NOT restored but gone through, cleaned up, made safe, running/driving... The last thing that needs done is the wiring. When I bought the bike none of the wiring was hooked up and it was direct wired from a battery to a DC coil to the points. I had no idea what alternator was in the bike, or if it was any good. Going by the engine # i thought it would have the 40W, I tried to test it and it seemed to be shorted to ground (going by the test for the 40w) i took the alternator off the bike and found out it has the 28W installed. It lookes to be in good shape, and my brother soldered on new leads. I am ready to refitt it on the bike and I need to make sure that all my parts match up, correct ign. advance, flywheel, ect. I also need to make sure I install the flywheel in the right position. I will be doing this in the next few days, and will have some questions for sure.

Also, Joe this is the post you were probly refering to when you were talking about building a rig to test alternator output.

PS...I wish there was a place on this site that all of the information on the different Ducati ignition systems could be moved to. It seems to be spread out under multiple and sometimes unrelated threads and difficult to search for. Other forums I have been a member on have forum subtopics to make that less of a problem. for example under the "Ducati Singles Talk" section (which is where 99% of all the posts to Motorscrubs goes) there could be subtopics like "Singles Electrics" "Engine & Cylinder Head" "Desmo Only" "Chassis and Suspension" "Paint and Surface Restoration" "Model ID." Ect...
Just my opinion.

[DewCatTea-Bob]

" the engine in the bike is #86687. "

____ I think that makes it a late-63 or early-64 model.



" I had no idea what alternator was in the bike, "

____ I assume it's an alt.model which has three wire-leads ?



" Going by the engine # i thought it would have the 40W, "

____ Indeed it OUGHT to be.



" I tried to test it and it seemed to be shorted to ground (going by the test for the 40w) "

____ With that older alt.model, only it's Red-wire should be grounded (to it's stator-plate).



" i took the alternator off the bike and found out it has the 28W installed. "

____ I assume you mean off the older engine ?
Exactly what leads you to believe that it's the '28-watt' alt.model ? ... (Your OLDER-motor is too old to have come-stock with that newer/1966-type alt.model !)



" I also need to make sure I install the flywheel in the right position. "

____ Did you happen to read of any related notes (which I've often posted on this w.site) recommending that ya place a mark on the flywheel/rotor-shank to indicate (and keep track of) the rotor's preset TDC-location, BEFORE pulling it from it's original set-place on the crankshaft ?
__ Have you checked your alt.rotor's magnet-strength yet ?

I wish there was a place on this site that all of the information on the different Ducati ignition systems could be moved to. It seems to be spread out under multiple and sometimes unrelated threads and difficult to search for. Other forums I have been a member on have forum subtopics to make that less of a problem.

____ I've particularly expressed this very-same idea before (within at-least a couple other threads [such as 'Website Requests'], in such a manor so as to pretty-much emulate the familiar PC.folders tree-structure), but it seems nobody-else has much cared, (until you/now). _ As it's simply been easier to just-merely go-ahead & start a NEW-thread (on whatever particular-topic),, especially since our search-engine is so apt to be an 'ALL or nothing' type of an affair ! _ So it's fairly admirable of you to have first ATTEMPTED to locate exactly what you desired to learn-of !


Hopeful-Cheers,
-Bob

[amartina75]

Bob,
Yes the Alt. has 3 wires, red,white,yellow. Both the 28W and the 40W have the same wires, correct? they are just wired differently at the coils.
Going by the engine number I believed it to be a 65 motor also, that is why I expected it to have the 40W alt. in it. That is why I performed the test for the 40W and believed it to be shorted to ground.
Since taking the Alt. off and finding it to be the 28W I found out that the test I had done meant nothing and the Alt. was likely no bad but just fine and needed only new wire leads soldered to it.
I will say that even after I had removed the Alt. from the motor I still believed it to be a bad 40W because I did not look at it closely enough to see the difference between the wiring of the 40w and the 28W system and there being no diagram of the 28W system in any book I have, or on this website, also lead to my misunderstanding. (and my general ignorance of all things electrical)
My brother identified it as a 28W alt. from a 66 scrambler.
I don't know the history of the scrambler or what any of the past owners have done, but it is clear so far that the Alt. in the bike now is a 28W and was likely removed from the original motor (which is not installed or running) (I did take the original motor apart to see what Alt. model it had and found it to be a 60W. with an aluminum flywheel and two yellow wires.)
No I did not mark the flywheel before I took it off, there were already black marker lines on the flywheel from the previous installer i guess. I was unaware at the time that the factory markings could be in question.
I did check the strength of the magnets, of each rotor I have the brass one and the aluminum one, they both seem strong and about the same.
Later today I need to check the number on the brass rotor and look at the advance mechanism again.

The following quotes are taken from earlier in this posting and seem to be most everything I need to be concerned about at the moment.
Mostly they are from Bob.
Bob if there is anything you care to add or highlight for me I would appreciate it.
I will need to read over this information several times for it to make sence to me, and I will likely have questions later.
thanks, Aaron

"there are indeed two types of advancer units...
_ the '28-degree unit', that's meant for battery-type ignition (& the 6-volt Red-coil), and which has a suitable cam-lobe profile (that keeps the points closed for a relatively short time), for getting the ign.coil properly saturated, for max.COLLAPSING magnetic-flux, to create a spark,,
plus,
_ the '18-degree unit', that's meant for the higher voltage 'shot' of DC (not AC!), into the Green-coil (which is not actually an "AC" ign.coil !), for making spark! And which has a cam-lobe profile that keeps the points closed for an extended time so as to prevent the ign.coil from being over-exposed to AC, and, to make the points suddenly divert the power-coil's flux-induced pulse of DC-juice into the ign.coil, so as to create an EXPANDING magnetic-flux, to create a spark !
The very short pulse of DC (which is the only part of the power-coil's AC-output that is responsible for creating the spark, in this case), does not last anywhere near the time it takes to "saturate" that ign.coil ! _ The so-called "magneto" system just doesn't work that way! _ As AC cannot really 'saturate' a coil."

"There was a time back in the late '70s when I had at least that many spare flywheel-rotors collected (within a single heavy-duty box)!
___ As you're no doubt aware, each flywheel has a 'timing-mark' on the front side of the brass or aluminum face of the rotor. _ The timing-mark is in the form of a short vertical line -(about 12 to 15mm in length).
(A good DUKE-mechanic knows to always take careful note of the exact position of that mark [in relation to TDC] BEFORE pulling the rotor from the crankshaft!)
__ After I began to gather a collection of these rotors, I then noticed that the 'timing-mark' was not always placed in the exact same position of the rotor, in relation to the placement of the magnets! _ (Which doesn't make good sense!) _ But it seems to me that the so-called "timing-mark" should be placed so as to EITHER indicate exactly where the rotor should be directly-aligned with the TDC-slot (in front of the tapered-shaft which the rotor is pressed onto), OR else, indicate the exact point where MAXimum magnetic-flux of the rotor is located. - (NOTE: In the case of the 4-pole rotor, there would be 4 such locations [spaced 90-degrees apart] where a mark could be placed to indicate the MAX-flux,, and in the prior case, in which The MARK could represent the required 'TIMING' to the crankshaft, there could be 2 locations [spaced 180-degrees apart) where the mark could be placed !
Yet it seems that Ducati did not consistently maintain either of these two logical locations to place their rotor-markings!! _ (As I mentioned in another post, it SEEMS that Ducati chose some method, (or lack thereof), of rotor MARKing-placement so as to totally confuse ANY unwise mechanic !)
___ Anyhow, could you please look-over your 4 to 8 rotors and make careful note as to how the 'timing-mark' on the face of each rotor relates to the position of the magnets on the back-side ?
To do that job, I myself would simply use a L-shaped straight-edge to carefully guide a scratch (made with a scriber-tool), so as to make an accurate extension of the 'timing-mark' around to the back-side of the rotor, to then make note of the scratch-mark in relation to the position of the nearest two magnets.
Completing this job on more than 2 (otherwise identical rotors), I'm sure you'll fine no rime-or-reason for the exact location of EVERY such timing-mark which you check. _ (And I'll be surprised if you happen to find more than 2 rotors with their marks located in the exact same relative position!)
__ Please be so kind as to let us all know, (and everyone else who can do so, as well)!"

"Dew Cat Tee Bob
I checked some mk3 and scr flywheels ie 40 watt till motor #92171, flywheel # 31 20 94 stator # 31 95 416
And my factory parts book says, from 92172 on(1966) they were scr models ie 28 watt flywheel #3195471, stator#31 95 472
The wheel 31 20 94 and a wheel 31 20 92 although they looked the same the timing mark was in between magnets,
and the other was at the beginning of a magnet, I believe they are only marked (timing mark) differently to fit a certain application,
and are really the same.
I have to look for the 31 95 471 wheel for my 1968 scr (Berliner sold leftover 1966 scr's and marked the year of the bike when he sold them)Title says1968 but it's a 1966. I can't set the timing till I find the 28w flywheel.
I'm glad you pointed this out, as I would have gotten the timing wrong, because I took flywheels off, and put them in a box a long time ago.I should have marked each with a marker as I took them off.
I have an unusual one, it's 5" wide 6 magnets,brass, and is marked with all the Ducati markings part #31 44 92,can you help me identify what it would fit.My master parts book only goes 1964-1966 model years, I need some one with a parts book from1960- 1963 to get the right magnetos for 1962 250scr and the1963 mk3, you're the man Bob Thanks a lot!!captpaul "

"Right! _ That's a common oversight!
I always tell others to be sure to add a scratch onto the rotor's taper-mount, directly above the keyway-slot, before pulling the rotor off (from any factory-stock motor), so that the original phasing won't be lost! _ But most forget to do it anyhow. _ Even I had never bothered to make any note (before I discovered that their original timing-marks were not actually trustworthy). _ As ya just assume that the factory had everything right to begin with!
But after comparing a fair number of otherwise identical alt.rotors and finding that their timing-marks were almost completely inconsistant (even from the same model-year), I then came to realize that I should not trust that the original-marks were actually set for the purpose of optimum ignition-timing, (or any consistant purpose what-so-ever). _ Thus the only answer that makes sense for the inconsistant marking placement (on at least some unknown number of rotors), is ERROR by those in charge of the process at the time.
So I had to try to figure-out which timing-marks were correctly placed and then determined that MOST were not properly located for optimum synchronization with the ignition's static-timing. - There is no other logical reason for placement of The MARK elsewhere on the rotor, (and even if there were, it seems that there was never any agreement about it that remained consistant).
Thus I had to come-up with an easy method to accurately place a replacement-mark in a LOGICAL location.
I'll explain it as soon as someone needs to know, (maybe sooner)."

_____ "This alt.stator has four poles and it's rotor also has four poles...
___ On the stator there are two separate coil-windings, each wrapped around it's own magnetic-steel core. - (Those cores are made of several layers instead of one solid piece, so as to reduce stray eddy-flows of electrons which may create magnetic fields that are not coherent with that which is desirable).
As is obvious, the two ends (of each of the two cores) are convex-curved so as to closely clear their concave counter-parts within the rotor ! _ These curve-cut protrusions -(from the coil-windings), of the stator-cores are the 'poles' of the stator,, and of course the center of the core-protrusion, is the center of the pole!
__ In the rotor there are four magnets arranged so that the rotor's four concave-curved magnetic-steel poles are magnetically-charged (by both magnets on either side of each), with either North or South polarity. _ The center of these concave rotor poles is the point where maximum magnetic-flux is most concentrated, (thus the pole's equivalent of a 'peak'). ...
__ So when the rotor's four pole-centers become directly aligned with the stator's four pole-centers, that then is the point at which maximum/peak-inductance of electromotive-force occurs. _ And thus the alternator then produces the 'peak' of it's power-pulse !
____ If the rotor was transparent, then it would be easy to check the alignment of all the pole-centers, without the need of any related reference-mark placed on the front-face of the rotor ! _ From the inside-back of the rotor, it's quite simple to determine where the centers of it's four poles are located and to place a centerline-mark (from the center of any of the four poles to the center of the pole that's 180-degrees across the rotor), on the in-side surface of the rotor !
Drilling two small holes on that added center-line from the in-side through to the outside-face of the rotor, would certainly provide perfectly placed reference-marks to be easily seen on the front-face of the rotor, so that a corresponding centerline-mark could then be placed outside where it could then be of some use ! _ But of course ya really wouldn't care to do THAT.
__ So how do ya suppose they figured-out at the factory their method for deciding exactly where to stamp some kind of a timing-mark which had some logical reason for being located where-ever it was to be, on the rotor's face? ... That's always been a mystery to me, as the question wouldn't really need to be asked, if ALL 4-pole rotor timing-marks had been consistantly located in the exact same spot on the rotor-face ! ... But I've noticed that the timing-marks placed on most 4-pole rotors are NOT found in the exact same location, in relation to the position of the rotor's magnetic-poles ! _ And when I have found any rotors which all happened to have their timing-marks placed in an identical location, THAT location, (other than those which had been placed at the zero-degree spot), wasn't really a LOGICAL place for any 'TIMING-MARK' (which could be easily aligned with some existing point or thing) !
Therefore I find much reason to doubt that there was always some logical reason for the locations where Ducati placed their rotor timing-marks. _ And in any case, these days ya really can't blindly trust that your duke actually still has it's factory-original rotor,, even if it's timing-mark was 'spot-on', and EVERY workshop-manual included ALL the FACTUAL degree-figure settings, so ya could then be sure you'd be able to correctly set YOUR exact model !!
____ Anyhow, if the rotor were to be pressed-onto the crankshaft with all the pole-centers exactly aligned - (which we shall refer to as 'Top-Peak-Power'), while the crankshaft is at Top-Dead-Center, (as would seemingly be a logical concept to do), THEN the alternator would of course produce the peak of it's power-pulses at every TDC (as well as every 90-degree arc-turn from TDC) ! _ And that would be just fine&dandy IF the ign.spark also always occurred at TDC ! _ But we know that's not the actual case... Because with most of these n-c Mk3 & SCR models (which use this type of 4-pole rotor), their (static) ign.spark is to occur at about 22-degrees before TDC (and advances from there to 39 to 41 degrees BTDC).
So therefore, from that seemingly logical point -(TopPeakPower aligned with TDC), the rotor must then be advanced (about) the same 22-degrees, by rotating the alt.rotor counter-clockwise 1/16th-turn (while the crankshaft remains at TDC) !
___ With the use of a 'right-angle' or a 'T-square', ya could guide a slight etch-line (to be placed across the outer circumference of the rotor), that's exactly aligned with any of the rotor's four pole-centers. - (I'd make such an aligned etch-line at all four such points.)
__ Next, (for most of us who don't have a degree-wheel that's a bit smaller than the rotor's diameter), ya can use a length of tape that's NON-stretchable -(like the clear nylon-reinforced type, [NOT masking-tape!] ), by tightly rolling it one complete turn around the rotor's outer-circumference, starting & ending at your chosen etch-line. _ After the length of tape has been cut to the exact circumference of the rotor, then pull it off and fold it exactly in half and cut it right on the half-way point... This will then provide two pieces/lengths which are each equal to 180-degrees of the rotor's 360-degree circumference! _ Then repeat that process (with either piece) to get a 90-degree piece,, then again for a 45-degree piece, and then finally one more time to get a 22.5-degree piece. _ (If done correctly, all half-pair pieces of tape should be exactly the same length!)
You'll need one 45-degree piece & one 22.5-degree piece.
__ Now with the rotor held like a clock-face that's faced toward you, and with your etch-line at the 12-O'clock position,, then place the cut-edge of your 45-degree piece evenly with your etch-line -(that's been made on the rotor's outer-circumference & aligned with one of the 4 pole-centers), and spread it over the rotor (like it was when ya made the 360-degree piece).
At the other end of this 45-degree piece, another (new) etch-line could be made which would then stand for Top-Peak-Power, (referred to as 'TPP', as I can't think of any official-name for it).
No matter which side of your etch-line -(which indicates a pole-center), your 45-degree piece is placed on, the TPP-line should be located exactly evenly spaced from the nearest two pole-centers of the rotor !
Now at that TPP-line/point, you place the cut-edge of your 22.5-degree piece of tape and spread it towards the 1-O'clock position,, and at it's other cut-edge (exactly at 12:45), you have now found one of four location-points on the rotor that should be aligned with the crankshaft's TopDeadCenter-line ! ...
You could extend a line from that point to the front-center of the rotor, so as to LOGICALLY locate a real 'TIMING-MARK' on the rotor's face, which could then easily be directly-aligned above the center of the notch on the tip of the crankshaft, or the drive-gear's key-way slot, (either of which should indicate crankshaft-TopDeadCenter). _ So therefore the created timing-mark would be as easy as possible to use, to help see that the rotor is properly phased with the crankshaft while pressing the rotor onto the crankshaft, (with or without the engine kept at Top-Dead-Center) !
____Somebody who quite well understands that the rotor must be properly phase-timed to the crankshaft so that the alternator then produces close to it's maximum-power during the time when the ign.spark is to be produced, may reason that since the ign.spark (of the duke-models in question here), occurs anywhere between 21 & 41 degrees BTDC,, that it would make sense to phase-time the rotor (onto the cramkshaft) so that the alternator would then offer it's peak-power right-in-the-middle -(at 31-degrees BTDC), of those ign.timing limits... Well doing that would no-doubt still work but, the reason for setting it near 22-degrees BTDC, is to make sure that the ign.coil get's as much power-juice as possible while at KickStarting-RPM, (as nobody really wants to have to kick-over the engine any harder than they really have to, in order to get the best spark for their effort).
And besides, as the engine's RPMs increase, so does the power-output of the alternator ! _ So that tends to cancel the loss which the advancer causes (when it makes the spark occur at a time when the power-pulse has not yet peaked) !
__ However for the older models which do not employ any auto-advancer, the phasing-job of the rotor can simply skip all the steps envolving the pieces of tape! _ As any etch-line that's been aligned with a pole-center (of the rotor's four of such), has located a place for a timing-mark that's close enough for those particular models, (as the 4 to 6 degree difference between THAT static-timing setting and THIS simple peak-alignment setting, is not significant). _ Perfectionists can use a small degree-wheel to see about making a more accurately located timing-mark, which should then be 4 to 6-degrees advanced just ahead of any of the four possible etch-line spots, (thus retarding the rotor just a small bit [compared to simply using an existing etch-line as your timing-mark] ).
However, I hope that those of you who choose to retain the stock no-auto.advancer set-up, either expect to most always get started-up by using the push-start technique, or else have your nees & ankles as tuff as humanly possible !
But if you are concerned about keeping all your involved body-parts from harm, then consider reading my post about installing a 28-degree auto.advancer-unit, (as I've suggested earlier within this same thread).

DUKE-Cheers,
DCT-Bob
Last edited by DewCatTea-Bob on Mon Mar 29, 2010 11:26 am,

[DewCatTea-Bob]

" Both the 28W and the 40W have the same wires, correct? "

____ Yes, the same three COLORS but, their White-wires are actually NOT "the same wires" ... as THAT-colored wire-lead doesn't serve the same purpose in both systems !



" they are just wired differently at the coils. "

____ Actually, it's not just the manor in which those alt.wires-leads are connected-up to the stator, but ALSO the alt.power-coils themselves are dissimilar as well.



" Going by the engine number I believed it to be a 65 motor also, that is why I expected it to have the 40W alt. in it. "

____ Indeed, that would be the expected case for a STOCK-motor.



" That is why I performed the test for the 40W and believed it to be shorted to ground. :

____ But actually, BOTH the 28w AND 40w stators have their lighting-coils' Red-circuit grounded to the stator-plate ! _ So your ohm-test will only show such a difference when testing the White or Yellow wire-connections.



" Since taking the Alt. off and finding it to be the 28W I found out that the test I had done meant nothing and the Alt. was likely no bad but just fine and needed only new wire leads soldered to it. "

____ To be more sure that your alt.stator is indeed a 28w.model (against the expected odds), it would be useful if you could post a picture of it,, (and then we could also check-out your new wire-lead installation-job).



" I will say that even after I had removed the Alt. from the motor I still believed it to be a bad 40W because I did not look at it closely enough to see the difference between the wiring of the 40w and the 28W system and there being no diagram of the 28W system in any book I have, or on this website, also lead to my misunderstanding. "

____ Indeed there has been plenty-enough confusion between the two DIFFERENT alt.stators and their respective electrical-systems.
The 28w.type system (on 1966 Scr.models) was borrowed from the 160-MonzaJr.model, so you can look at IT's system-layout if you wish to employ the same stock 28w.type system.



" My brother identified it as a 28W alt. from a 66 scrambler. "

____ It would be reassuring if we could know exactly how he had made that determination (seeings-how it [last] came from a motor of which it wasn't original-stock).



" but it is clear so far that the Alt. in the bike now is a 28W and was likely removed from the original motor "

____ Assuming you got both motors from the same source, then that does seem like a likely scenario.
__ That you've already reinstalled that alt.stator back-into your intended Duke-motor apparently must mean that you have already chosen particular plans for the electrical-system which you wish for your project-Duke to employ, is that correct ?



" (I did take the original motor apart to see what Alt. model it had and found it to be a 60W. with an aluminum flywheel and two yellow wires.) "

____ If that 60w.stator happens to be stock to that motor, then it's not a Scrambler-motor and should thus-then be attached with FOUR screws, however if that stator is only attached with just 3-screws, then that motor is probably indeed likely the Scr.motor as you've expected.



" No I did not mark the flywheel before I took it off, there were already black marker lines on the flywheel from the previous installer i guess. "

____ Then hopefully, (if you didn't check to make sure before the rotor was pulled), those black-lines were marked so as to indicate the original factory-set TDC-location (rather than whatever/who-knows-what).



" I was unaware at the time that the factory markings could be in question. "

____ After past tinkerers have swapped-around alt.rotors between various model-year productions, ya can then of-course no-longer trust the related info that's stated within the workshop-manuals ! _ And on top of that, (even without such past juggling), I've found that a good number of alt.rotors do-not have their timing-marks consistently stamped in the same location (with respect to the magnet-arrangement),, thus leading to the question of WHICH is the CORRECT location !



" I did check the strength of the magnets, of each rotor I have the brass one and the aluminum one, they both seem strong and about the same. "

____ I'd say that if a female needs to use both her hands to pull it off from a refrigerator, then it's magnets are probably strong enough.
__ Do both of those rotors have just 4-magnets ?



" Later today I need to check the number on the brass rotor and look at the advance mechanism again. "

____ The rotor-number shouldn't matter in that regard.
Can you post a picture of that AAU ?



" I will need to read over this information several times for it to make sence to me "

____ If you come-across anything that at all seems unclear to you, then please quote that wording, and I will then gladly restate it's intended meaning in other-words.


Duke-Cheers,
-Bob

[amartina75]

Bob,
Sorry I do not have any pictures of the stator or the advance unit. I did not take the points off so I am just going by the shape of the points cam lobe.
the one I have is the 18 deg. type as the cam lobe is mostly closed, with a short lift duration.
As you said the stators are more different then I sugested, I did oversimplify that, what I meant was they look the same at a glance.
The 28w has both coils connected to ground.
Yes I plan on wiring it like a stock 66 scrambler.
I do not have a suitable coil though as the bike was set up to run total loss, it had a DC coil on it. so I need to find a coil I can use, hopefully cheap.
I was thinking of using a coil made for an old honda CT90, but I am not sure which one is best.

I guess my main confusion is with the factory mark on the rotor and in what position it should be placed in relation to the keyway on the crank at TDC.
Is there really good reason to believe that the mark is "wrong" or off by enough to really cause concern?
There seems to be only a few ways Ducati could have marked the rotors at the factory.
They could have placed the rotors on a jig that held them strictly by the shape of the inside of the rotor.
They could have used a jig that located them by the magnetic field of the rotor itself.
Or they could have tested every rotor they ever made for optimum output. (very unlikely in my opinion)
So I don't see how there could be that much difference between the marks on the rotors. If they used some sort of magnetic field allignment then you would expect there to be some variation.
If they used just a keyed type jig there would have to be clearance for the rotors to be placed on it and removed.

Going by the Factory Workshop Manual. the Flywheel Position (of the factory mark) for a Scrambler or Monza Junior with a 28W alt. and the 18deg advance unit is 32-36deg. counterclockwise from the 0 deg(or 12o'clock) position. Is that correct? Also are you suggesting that it should be closer to 32deg. or less? to lessen kickback, and ease of starting?

As a general note about me, my understanding of electrical systems is VERY limited. I am an artist/blacksmith/metal fabricator, and very good at it if I do say so myself.
So if I say anything that is completly absurd, forgive me. I am the learn as you go type when it comes to things like this.

Thanks, Aaron

[DewCatTea-Bob]

" Yes I plan on wiring it like a stock 66 scrambler. "

____ That model's electrical-system was quite sufficient, however it's headlight leaves a bit to be desired below 3000-RPM.
If you later wish to do anything about that, we can discuss options later.



" I do not have a suitable coil though as the bike was set up to run total loss, it had a DC coil on it. so I need to find a coil I can use, "

____ I believe the only difference between the Red-labeled DC-ign.coil & the Green-labeled magneto/non-battery type ign.coil, is their 'turns-ratio'. _ Many others have mixed-up these stock-coils and found that either type works with either ign.system. _ So the ign.coil you happen to have will probably work well enough.



" I was thinking of using a coil made for an old honda CT90, but I am not sure which one is best. "

____ That should also work about as well, (and possibly better than the Green-coil).



" I guess my main confusion is with the factory mark on the rotor and in what position it should be placed in relation to the keyway on the crank at TDC. "

____ All Ducati ever had to do was ALWAYS consistently place the timing-mark so that the magnet-rotor would be properly-located whenever that stamped-mark is DIRECTLY-aligned with the TDC-point of the crankshaft, (so that then nobody would ever have to be concerned with making-sure that the particular alt.rotor which they happen to have is mounted with IT's particularly-placed timing-mark oriented at whatever number of degrees [in relation to TDC] is necessary for it's magnetic-field to be properly align-timed with the ignition-timing). _ And that's assuming ya know-for-sure WHICH particular motor-model the alt.rotor in question originally came from, (and that the related workshop-manual data is absolutely correct as well).
__ The related point made by CaptPaul pretty-much confirms what ya need to be warned about.



" Is there really good reason to believe that the mark is "wrong" or off by enough to really cause concern? "

____ There's certainly a better than fair-chance that a factory-placed timing-mark was actually stamp-located in the (seemingly quite nonsensical) position which it was meant to be ! _ However, it's been my experience, (long-ago back when most Duke-motors were still factory-virgin & untouched by tinkerers), that at-least a small number of the alt.rotors did-not happen to have their timing-marks stamp-placed in the very-same location-spot as indicated it should be (within the related workshop-manual listing).
__ And YES, the amount of difference (in degrees) between the varying locations of the timing-marks (from one rotor to the next) can make a VERY-substantial difference in how much power the alternator happens to be able to produce at the particular time when the ign.system happens to most need it !
Many, (but certainly not all !), alt-rotors happen to have their timing-marks not-only not place-located in any logical-reference to TDC,, but also not even in logical-reference to the magnet-locations, or even the intended motor's ignition-timing !
In-short, the timing-marks on a substantial number of the alt.rotors, simply ought-not to be trusted ! _ And the only way to discover if the timing-mark on YOUR particular alt.rotor is close-enough, is to check-it-out for yourself & see.



" There seems to be only a few ways Ducati could have marked the rotors at the factory. "

____ Since these mag.rotors have 2-pairs of poles, they could've had 2 (or even 4) timing-marks LOGICALLY place-located on the rotor-face,, either so as to indicate the rotor's magnet orientation-arrangement, (seeings-how ya can't see-through the rotor-face, to see directly for yourself), or, so as to indicate the rotor's required location-orientation DIRECTLY to TDC.
But instead, it SEEMS that Ducati chose to stamp their timing-marks in such a manor so as to totally-BEFUDDLE anyone who ever dares to pull-off a factory-set alt.rotor (and hadn't made-note of exactly-how it must be reinstalled). _ (I of-course realize that must be ridiculous but, it certainly seems that way.)
__ Can you post a picture of the back-side of your alt.rotor ?
If so, I could then SHOW you all the various points of concern, which would really help you to understand & grasp all the concepts involved.



" Going by the Factory Workshop Manual. the Flywheel Position (of the factory mark) for a Scrambler or Monza Junior with a 28W alt. and the 18deg advance unit is 32-36deg. counterclockwise from the 0 deg(or 12o'clock) position. Is that correct? "

____ Well, that's as good of a GUESS as any ! _ Cuz if going by THAT (or any other guess), CHANCES-are that the alternator will just-happen to make some random amount of power which MIGHT happen to be enough to work-out okay.
So you could give that setting a try & see (with fingers crossed).
__ But if you really desire to make-sure that you have max.power-output occurring during kick-start RPMs, THEN you need to eliminate any such chance-work !



" Also are you suggesting that it should be closer to 32deg. or less? to lessen kickback, "
____ Not sure exactly what you mean by "it".
The ignition-timing ought to be set about 18 to 21-degrees BTDC, however 3 to 6 degrees even less, would decrease the likely-hood of kick-back.



" and ease of starting? "

____ With the alternator optimally set, the best possible ign.spark can then be available for kick-starting,, otherwise you either have to kick a lot harder, or-else resort to push-starting.



" my understanding of electrical systems is VERY limited. "

____ That's okay, I usually always try to explain electrical-stuff so that ya don't really have to be an elec.tech in order to understand.


Hopeful-Cheers,
-Bob

[amartina75]

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Bob here are the pictures you requested.
there are three lines drawn on the rotor. one is the factory marking that is highlighted.
one is marking where the book specs it to be(32-36deg from the stamped in mark)
the other mark is where it would be if I followed your suggestion.
You can see where these marks line up on the back as well in the pictures.
looking at the rotor from the front or back, the two marks that are close together(about 10deg seperation) are where the book suggest and where i believe you suggest timing the rotor at.
these marks end up being 32-36 deg. for the book
about 42deg for your suggested mark

If i followed what you were saying, and i think i did. You said basically to find the center of the magnet(by going 45deg from the center of the pole in the rotor) but thats just the center of the magnet so no need to really start at the pole, just start in the center of the magnet.
then you said to go from there to 22deg counter clockwise and that is the place to set the rotor at TDC.

you can see in the pictures where these marks line up on the back of the rotor your mark is basically right at the line where the magnet meets the steel plates.

I will say this, my brother has taken apart every motor he has with a 28W magneto and they were all the same. (2 scrambler motors and a monza jr motor)
the factory mark was in the same place as mine and they were all set at about 32-34 deg.

I am inclined to reinstall mine "by the book" unless you can convince me there is good reason not to.
I don't really like not following directions or manufacturers specifications when i do things. I like things to be stock and as the designer/engineer intended.

One other thing when the stator is installed on the engine it is tilted a few degrees from level, so that it lines up with the center line of the cylinder not a line at 12 o'clock.
placing the rotor on the motor where your set timing mark is at 12 o'clock would then essentially move that mark in realtion to the stator a few degrees.

Let me know what you think,
Thanks,
Aaron

[DewCatTea-Bob]

" Bob here are the pictures you requested. "

____ Actually more than requested... thanks !



" there are three lines drawn on the rotor. one is the factory marking that is highlighted. "
____ Right, that's indeed the standard stamped-slash that the factory intends as the 'timing-mark'.



" one is marking where the book specs it to be(32-36deg from the stamped in mark) "

____ Well-done... That added ink-mark is where the workshop-manual would expect the TDC-point of the crankshaft to be aligned-up with.
That particular position would have the crankshaft holding the alt.rotor so that the alternator then produces it's max.power-pulse about 12-degrees BTDC.



" the other mark is where it would be if I followed your suggestion. "

____ Yes, you obviously got that all-right.



" You can see where these marks line up on the back as well in the pictures. "

____ Very-good ! _ As it helps a lot to see how everything lines-up back/behind there !



" looking at the rotor from the front or back, the two marks that are close together(about 10deg seperation) are where the book suggest and where i believe you suggest timing the rotor at. "

____ You are correct, that difference is due to my recommended-setting being nearly 10-degrees further advanced.



" these marks end up being 32-36 deg. for the book "

____ By that, I believe you mean that YOUR (expected-TDC)- mark is about 34-degrees retarded-back away-from the location-point of the factory-mark.



" about 42deg for your suggested mark "

____ That amount of difference would have to be measured with a degree-wheel, to be certain,, but if the magnets & the core-plate sections span the exact-same amount of arc-space, (as they appear to),, then my semi-suggested TDC.timing-mark location is pretty-much an even 45-degrees retarded from the (inexplicably located) factory-mark.



" You said basically to find the center of the magnet(by going 45deg from the center of the pole in the rotor) but thats just the center of the magnet so no need to really start at the pole, just start in the center of the magnet. "

____ Well when you put it THAT way, it seems like a silly/extra step,, which indeed it would be, with the use of a degree-wheel. _ But with the use of cut-halves of tape to represent pre-set amounts of degrees to help with location-points to be seen on the FRONT-side of the rotor, it helps to be more certain by starting with the hole that's more obviously centered (in the core-plates).



" then you said to go from there to 22deg counter clockwise and that is the place to set the rotor at TDC. "

____ I don't think I put it that way, without referencing which side the rotor was being faced from.



" you can see in the pictures where these marks line up on the back of the rotor your mark is basically right at the line where the magnet meets the steel plates. "

____ When I wrote-up those instructions, I didn't have an alt.rotor to look-at so I could see that obvious happenstance.
(It's nice that we now have the picture which I've posted.)



" I will say this, my brother has taken apart every motor he has with a 28W magneto and they were all the same.
the factory mark was in the same place as mine and they were all set at about 32-34 deg. "

____ As I had indicated, that type of occurrence ought to be expected more-so than not.
It's just not something you ought to trust 100%, especially when there's been a fair chance that an older alt.rotor (with a varied factory-mark) may've had an opportunity of getting mixed-in.
__ I now believe that all your alt.rotors taken from their 28w.stators, have their timing-marks specifically stamped just as the factory had meant for them to be located.
__ It would be additionally interesting to check & see if all of those (supposedly identical) alt.rotors had their factory/timing-mark extra-particularly located with N & S poles also identically placed on either side, (or if instead, their timing-marks were possibly NOT particularly-placed in-regards to the surrounding magnetic-polarity). (?)



" I am inclined to reinstall mine "by the book" unless you can convince me there is good reason not to. "

____ Now that we know what's-what with YOUR alt.rotor,, then if you go-ahead & do so, I expect that intended setting will work well enough. _ However, your alternator will then never be able to provide it's available max.power-output to the ignition-system, (unless you chose to retard the static-timing to about 14-degrees BTDC).



" I don't really like not following directions or manufacturers specifications when i do things. I like things to be stock and as the designer/engineer intended. "

____ In this case (with a rotor that doesn't seem to be miss-stamped), I believe you're good-to-go with that reasoning. _ However, if you happen to error, then best to error TOWARDS my recommendation (rather than away-from). _ In other words, set your rotor so that the factory-mark is 37-degrees advanced from the crankshaft's TDC.position.
__ When I came-up with the method of setting the alt.rotor so that it's max.power-output would occur at 22.5-defrees BTDC,
that was merely meant to help someone get their rotor timed in the center of the ball-park, (so to speak).
However for best possible energy-transfer to the ign.system, I'd most recommend that the rotor-setting be set at 39 to 42 degrees retarded from that factory-mark on your alt.rotor. _ That way, the power-coil will then become more flux-saturated before the points open, and yet there'll still be as much as 3-degees of peaking-power to fully excite the ign.coil, (thus provide the strongest ign.spark).



" One other thing when the stator is installed on the engine it is tilted a few degrees from level, so that it lines up with the center line of the cylinder not a line at 12 o'clock.
placing the rotor on the motor where your set timing mark is at 12 o'clock would then essentially move that mark in realtion to the stator a few degrees. "

____ This is a DETAIL which is the kind of thing which many people just don't think to get cleared-up to be ironed-out & mutually-understood ! _ So it's quite thoughtful of you to have taken note of such.
Most others would simply ASSUME that 12-o'clock is the same as TDC,, which in actual-reality is not the case with Duke-engines, as the cylinder is tilted-forward 10-degrees from vertical.
So-therefore, we will now be SURE to sensibly make it clearly-realized that for any reference to 'o-clock' reference-points, will be with the agreed-understanding that '12-o'clock' does indeed refer & equate to 'TDC' as being one & the same (for OUR particular purposes),, (since referring to TDC as actually being truly at 11:58:20-o'clock, [as 10-degrees = 1min40sec], would thus-then make our discussion even more complex than it already has to be !).


____ I've edited your picture showing the back-side of your alt.rotor, and added colored lines at points which have some pertinent-relevance to the physical-layout of the rotor.
__ The red-line & white-line indicate the 2-pairs of magnetic-pole peak-centers. _ When those center-points become directly-aligned with the equivalent center-points of the core-plates of the power-coils,, THEN, PEAK-power is generated from the stator-core power-coils.
(So if Ducati had chosen one of those four points [where either a red or white line is shown located], then that's one 'LOGICAL' place to have located a timing-mark.)
__ Concerning the blue-lines,, considering the orientation of the stator's power-coils (being tilt-placed at right-angles to the cylinder), the blue-lines are centered right in between the peaks of the magnetic-poles. _ So IF any blue-line were to be aligned with TDC, then of-course peak-power would be available THEN, at TDC -(which of-course is 20 to 23 degrees too-late for static-timing, [and even-further TOO-late for full-advance ign.timing] !).
(If Ducati had chosen one of those four points [where a blue-line is shown to be located], then such a location-point would've been a LOGICALly-chosen spot to place a timing-mark at.)
__ In order for peak-power to be available when it is MOST needed, the alt.rotor needs to be advanced so that as much power as possible is provided when the ign.points open at the static-ign.timing -(which is set between 20 & 23 degrees BTDC).
If the mag.rotor is installed onto the crankshaft so that the location-point of any of the green-lines is directly-aligned with TDC, then peak-power will always be produced at 22.5-degrees BTDC, (which is just a tad-bit early for static-ign.timing which happens to be set at 20-degrees BTDC, because it's slightly more optimum for the power to still be building-up towards peak-power [rather than falling-downward from peak] when the points open). _ But even-so, this setting will certainly provide a hotter ign.spark than that resulted by employing the factory-mark's intended rotor location-point, (as it is too retarded for the intended static-ign.timing of models which employ the 18-degree AAU).
(If Ducati had chosen one of those four points [very near where a green-line is shown to be located], then such a location-point would've been the MOST 'logical' place to locate THE timing-mark, cuz then it would be so very-simple to directly-align the mag.rotor with the crankshaft's TDC-point !).
__ Why Ducati never settled on just one LOGICALLY-located timing-mark location (for ALL of their alt.rotors), is a certain mystery.
At this point, it now ought to be understood that the chosen location for the factory-mark in question, has no logical rime-or-reason for it's particular location-point.
If anyone can come-up with any fairly sensible reason for why Ducati may have stamped their factory-mark in the particular location which they chose, then it would be interesting to consider.


Duke-Cheers,
-Bob
UPDATE - I've now added a yellow-line to show the most ideal location-point for the alt.rotor to be 'timed' to the crankshaft. _ That location is about 40-degrees from the factory-placed timing-mark, and should allow the alternator to provide it's maximum-available power-output for kick-starting, (not to mention a stronger spark at higher RPMs as well !).
If an elongated timing-mark is added to the rotor-face where the yellow-line is indicated, then it would be most simple to directly-align that added line with the TDC.point of the crankshaft, and thus-then have the alt.rotor optimally 'timed' with the ignition-system (at kick-staring speed).

[amartina75]

Bob,
Your original argument that Ducati did not mark all of their rotors the same is true. My brother found a rotor, possibly from an older 40w Alt. that is not marked in the same spot as the 26W rotors.
So all together that is 6 rotors checked, 5 were the same as mine pictured and one was different, the one pictured below.
It doesn't make sence to change the location of the mark on the rotor, when they could have just changed the angle they specified to install the rotors at.
In fact if they were going to mark the rotors differently for every application, why not just place a mark where they actually wanted them installed at.

So if anyone didn't know where their particular rotor came from they should check the location of the mark before they install it. (considering all the rotors I see on ebay selling without the matching stator) If they follow your instructions they could install the rotor correctly without any factory mark at all.
However there is one thing that you are overlooking, or haven't made clear so that someone reading this post in the future would have trouble.
As I mentioned before, the rotor is placed over the stator so that you gain 10deg. of advance (if you follow the instructions in the book)
the stator is alligned with the offset in the cylinder. (see pictures below)
If you mark your rotor by "geometry" and you do not account for this difference (like the factory did) you will have two different marks when in fact the marks should be the same.

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