Recharging 250 MK1/MK3 Magneto

[velotwin]

I'm looking to re-magnetize my magneto on my Diana MK3. Does any one have any information on what the original measuring specs are on the magneto/flywheel on a 6v. set up? Any information or advise would be appreciated. Thanks, Ian

[JimF]

Hi Ian,

Do you have a mechanism (or access to one) that can re-magnetize the magneto? I did not realize there would be specifications for the process, or are the specifications for how to set up the machine so as not to blow out the wiring with an over-current situation?

Maybe Henry Hogben in Canada would know. I will try to contact him and report back.

Jim Franzen

[DewCatTea-Bob]

____ I'm somewhat of an expert on the electrical-systems of all DUKE-models...
What you actually have is technically not an actual "magneto", (although it could be said you have a magneto-type ignition set-up),, nor is it actually "6" volts!!
What the older narrow-case Mk3s have, is a '40-watt' alternator ! - Although it will make exactly 40watts only at a particular RPM, (depending on load).
(Mach-1s have the '60-watt' alt., & 6v-battery type ignition!!)
__ Anyhow, why do you think the magnets in your flywheel need to be strengthened?
Was the flywheel dropped hard a number of times, or was it in a fire? _ Or did some dummy hook-up a battery to your alt.leads? ... (Any of which could weaken the magnets!)
(I'll bet you've been trying to figure why the ignition or lights are so weak, right?)
You might (likely) find that after all the trouble, you'll still have what-ever issue you're trying to solve.
Do you have another known to be good flywheel and a good N/S-compass?
__ Also, before we continue, please confirm that you're referring to a stock narrow-case Mark-3.


DUCATIly,
-Bob

[velotwin]

Yes Bob it is an original MK3 narrow case, the bike is set up all stock, and is a U.S. Model, hence no battery, horn, keyed ignition etc. Without getting into a long detailed discussion I would like to know if there is a specific measurement given to the magnetization of a Ducati 250 magneto, I know that over time the magneto can weaken, this would in my opinion seem to cause inadequate or infrequent running issues as it relates to spark,(as well as other areas such as lights.) The regulator can also have issues, bot I don't believe this one is bad. The motor was split for the first time during restoration, so the magneto assembly was carefully stored before reinstalling, the bike will run, but some of the symptoms are that it will fire up the first time, but afterwards will not produce a consistent spark, or it will be very weak, I have rewired the bike several times without success, so I want to go thru it completely, starting with the Magneto. If you have any specifics on this subject I would appreciate. Kind regards Ian

[DewCatTea-Bob]

" The regulator can also have issues, bot I don't believe this one is bad. "

____ Well Ian, first, I feel compelled to point-out that I really do consider myself to be a Top-Expert on the stock electrical-system of any Mark-3 (or even a Mach1)! _ And while odds are that you will not find anyone who actually knows more on the subject than myself, there's still a chance that there's somebody out there who may know some minor detail which I'm not aware of.
__ Now the one thing that stands out most obviously, is that you mention a "regulator" !? ...
Well sir, stock narrow-case 'Mark-3' models never came with any such device! _ Nor does any 'stock' narrow-case Mark-3 have a ready-made way to make any use of any such Ducati-made part, (without adding a battery)!
So if you really have any such device somehow connected to any of the otherwise stock Mk3 wiring, then that should be the very first area to investigate!!
__ So let's please get to the bottom of that strange issue, asap!
Please give me a description of that item which you think is a "regulator", and what wires it seems to be wired-up to on your "stock" Mark-3.
__ Also, do you know any of the history of this particular Mk3 of yours?
As there's a chance that it my be one which has a modified electrical-system installed by myself (in the '70s or '80s), or by someone who I had instructed on how to convert it so as to make some use of a battery.
____ NEXT, you mention that you had the "magneto" - (I assume you meant the flywheel & perhaps also the alt.coil/stator assembly !?), removed from the motor...
Well whenever the flywheel is removed from it's original place on the crankshaft, then it really needs to be properly reinstalled back 'IN-TIME' with the crankshaft!! _ And that's not really a job that just anybody who can turn a wrench, can accomplish exactly correctly! ...
__You see when trying to start the engine, the alt.coil which powers the ignition must be at almost peak-power output at the point when the contact-points open at 22-degrees Before Top-Dead-Center!
Since it's indeed possible (but not really too likely), to install the flywheel-rotor at some point at which the power-coil happens to be making ZER0-watts of power-juice when the ign.points open at 22-degrees B.TDC, you can then understand how a random installation of the flywheel onto the crankshaft, would probably likely cause the power-output to happen to be insufficient at some point when it's still needed. _ Thus possibly not producing any useful spark at some point between static-timing & full-advance !
__ Now I gather that you've been getting an ign.spark that's adequit enough at starting-RPM, to get the motor going... Yet I also gather that as the RPMs climb, your ign.spark becomes unsteady & unreliable.
Well if so, then what's happening is that your Mark-3's 18-degree advancer-unit is advancing the timing at which the contact-points are opening, in relation to the angle of the crankshaft ALONG WITH THE FLYWHEEL-ROTOR! _ Thus a spark is trying to be created at a time when the alternator's power-coil is not producing sufficient power-output for the job!
__ If such an incorrect rotor-timing setting is indeed the actual cause in this case, (and it very likely is), then the SETTING of the flywheel-rotor is either a few degrees too retarted or way too far advanced, (on the crankshaft)!
In such a case as this, the flywheel-rotor should be pulled and then either* be ADVANCED -(rotated counter-clockwise) between 9 & 18 degrees, or, be RETARDED -(rotated clockwise) between 72 & 81 degrees, and then reset back onto the crankshaft,, so as to move the required power-pulse closer to the optimum timing for creation of the ign.spark at ALL RPMs. _ (*NOTE: The only difference as to whether the alt.rotor is advanced or retarded [the suggested amounts], is the DIRECTION which the 'spark' travels between the two electrodes of the spark-plug! - [Which doesn't much matter.] )
___ For anyone who is already smart enough to be quite aware of the alt.rotor-timing issue, I likely still have some news for you! ...
The original timing-marks (which are stamped-on the rotor-face for the purpose of addressing the issue of proper alt.flywheel-rotor timing) of the various flywheel-rotors, were not always placed in the proper spot on the rotor ! _ And also, the degree-figures given for the various rotor-settings of the various models & engine-#s (within most publications), are not to be trusted either!! - (Likely only due to timing-marks having been incorrectly stamped [on an unknown number of rotors] in the first-place.)
I've been a technician in this field and have found this questionable timing-mark issue to be the case in more than just a few instances!
The whole method (or lack thereof) which was used to place the original timing-marks on the flywheel-rotors, (for assisting with the properly 'timed' installation of the alt.rotors onto the crankshafts), was seemingly done in such a manor so as to totally-confuse the average-joe wrench-turner !
I myself have come-up with a relatively simple way to logicly place a correctly located timing-mark for the flywheel-rotors! _ (Putting it in print will be a bit of a challenge though.)
__ Anyhow, so while the strength of the magnets in the flywheels certainly don't get stronger with age, they really don't (under normal circumstances) weaken enough to be the likely cause of troubles such as that which I gather you've been having. _ Yet, that which I'm directing you towards, most definitely is (a more probable cause for such)!

____ After I get your reply to work with, I'll then be better able to help you get to the actual solving of your power problem!
If you think that I'm not on the course which you wish to go with concerning what your actual issue is, then I ought warn you that you'd not be the first to blame the magnets in the flywheel, and also end-up making them worse after attempting to make them stronger!
__ I still have more info about this subject, and will part with more in due course.


DUCATIly,
DCT-Bob

[JimF]

Hi Bob,

The DC bikes had a generator and a regulator. Mark 3s got the magneto system and Mach 1s got the generator system.

The regulator is a small black box that resides under the seat. It has four diodes for rectification, a fuse, a capacitor and several coils of wire around a common iron core. There was a day ten years ago where I understood how coils of wire can perform the task of DC voltage regulation. I have slept since then and forgotten the theory as to how it works, but in the 1960s before the advent of solid state regulators this was a viable solution that did not require vacuum tubes.

I have thought about designing a solid state regulator to fit in the black box and replace the coils of wire. The trouble is there would be very little market to recoup the development and NRE costs.

Jim

[DewCatTea-Bob]

" The DC bikes had a generator and a regulator. Mark 3s got the magneto system and Mach 1s got the generator system. "

____ Jim, now it seems that I must make my previously-stated point again... I'M (pretty much) a TOP-EXPERT on ALL DUKE -(single-OHC) ELECTRICAL-SYSTEMS!! _ Thus there is next to nothing that you or anyone else could tell me that I'm not already aware of !
__ I'm afraid to point-out that you have just posted somewhat misleading info, Jim ! ... As no Duke has ever come factory-stock with a DC "generator" !
That which you referred to, is actually an 'ALTERNATOR' ! ...
It's rated at "60-watts", and offers AC-type power at a frequency equal to 3 x RPM,
(thus for example, at 6000-RPM, the 'alternator' of the Mach-1, [& also narrow-case Sebring, & post-1962/narrow-case 5-speed Monza models], creates it's AC at 300Hz,, while the Mark-3 & SCR model's "40-watt" alternator creates 200Hz AC, at that same RPM.)
____ Concerning that rather heavy black-box under the seat of the battery-powered n-c models, it is indeed a unique & clever rectifier-regulator unit !
It contains two 'half-wave' selenium-type power-rectifiers (for the charging system), plus a pair of low-power diodes, (intended for running the charging-light).
What's really unique about it (I believe), is that it makes use of counter-electromagnetic-flux type force to repel, (rather than waste to ground), excess potential power offered by the alternator. _ That's pretty neat! _ As there's then no big waste of power !
So that particular black-box (with it's unique type of power-regulation method, known to be referred to as 'saturable-reactor'), is not actually a 'VOLTAGE-regulator', since it merely works by inducing opposing/counter-flux against that of any excess power, (which is not being consumed from it's main -[gray] output).
Thus it's ultimately the condition of the battery that determines the actual voltage which the system has. _ (So be sure to use a battery that's in standard-condition, or-else the system may experience issues!)
And so also, where-ever you may read "6 volts" output, stated by Ducati,, that's just a 'rating', (if not out-right misinformation) !
__ One of that black-box's drawbacks seems to be that the greater the charging-current which the battery will accept (passing-into & through it), the greater is the regulator-circuit's effect on cutting alternator-output, (thus the slower the rate which the battery is fed charging-juice). _ So in the case when the battery is low & capable of consuming much charging-juice, it will then take that extra-much longer for it to get the needed juice for getting fully charged-up. _ (So never hook-up anything else directly to the battery/red-wire, other than the black-box itself !)
I don't really fully understand exactly why Ducati did the Red coil-circuit reversed (opposite from that of the Gray coil-circuit's effect), so that the more charging-juice the battery accepts, then the less alt.power is allowed into the system...
It could be that there's another level of theory which only the designer of the black-box could fully explain, or maybe it's merely the intended (compromise?) method for keeping the battery from getting overcharged, (although the stock-sized battery should not be so easily overheated by the relatively weak alt.output).
I suspect that it might've had something to do with the general acceptance in Italy (back in those days), that motorcycles were not ridden for long periods at night, (outside of city lighting), as they were expected to do during the daylight.
__ Other than charging it's battery more slowly (when it's most needed), it's quite a good system, (just extra heavy) !
____ Ducati actually only takes advantage of just half of the juice/power-potential that's available from their 6-pole alternator ! _ That's because of the following facts...
It has two separate alt.windings, each wrapped around just 4 of the 6-poles. _ Those two windings are oppositely wound so as to have opposite output polarities. _ Since both windings have one of their ends grounded (together on the stator-plate), it could be thought of as a center-tapped single-winding, (but actually that's not exactly the case).
Since each (of the two windings) is merely 'half-wave' rectified, the output from that black-box APPEARS to have a full-wave type output ! _ But it's actually two separate, out-of-phase half-wave DC-outputs ! _ (And so the opposite/negative half-waves of each alt.winding are left un-consumable !)
__ Since the early '70s, I've been gutting those black-boxes and replacing the fancy guts with full-wave bridge-rectifiers. _ That along with a fairly easy modification to the alt.stator, plus a 12-volt battery,, then allows for a fairly-good standard 12-volt system.

____ Anyone with any questions about this or any other DUKE electricals, just ask me! _ No need to trust anybody else, as you'll likely just get misinformation elsewhere.


Your Trusty & REAL Duke-Expert,
DewCatTea-Bob

[Eldert]

Hi Ian

do you have the correct advance unit for the flywheel / magneto ignition ?
the cam that opens the points has a differend profile to saturate the AC coil
i have one here from a 450 RT . the number on the a unit is AA367B
did you try another condensor ?

Eldert

[DewCatTea-Bob]

" do you have the correct advance unit for the flywheel / magneto ignition ?
the cam that opens the points has a differend profile to saturate the AC coil
i have one here from a 450 RT . the number on the a unit is AA367B "

____ Well Eldert, 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.

____ This is some of the additional info which I had mentioned in an earlier posting.
This new post which touched on this aspect of the subject, was what drew this additional portion of the info, out of me here.

GoodCheers,
-Bob

[DewCatTea-Bob]

_____ Today I noticed one of the 18-degree ign.advancer-units -(as is stock in the n-c Mk-3 & Scrambler models), sell for near 50$U.S. on eBay! ...
I really can't justify people bidding so high for that model! _ The only reason I could justify for installing that type of ign.advancer, is to make an otherwise incomplete 'restoration', totally 100%-complete!
Otherwise I'd always choose to replace that original unit with the 28-degree unit -(which was used on all models that employed battery-powered ignition).
Here's why...
__ Because that 18-degree unit requires that the STATIC-advance setting be set about 10-degrees more advanced, which increases the likelyness of a KICK-back while trying to use the footstart-lever. - (And extra-hard kick-backs are quite hard on the related internal-parts, not to mention one's foot!)
Also, the cam-lobe of the 18-degree unit was designed to allow the contact-points to ground -(therefore waste!) most of the power-juice that's produced by the involved power-coil -(of the alternator)! _ (The cam was designed that way because the ign.coil only requires 1/8th of all the [+ & -] pulses of DC-juice which the alternator supplies, and if most of that AC-juice were allowed to pass through the ign.coil [instead of wasted to ground], then the ign.coil would get way too hot!)
__ So I suggest to anyone who needs another advancer-unit (for their DUKE-model which used the 18-degree unit), that they consider using the 28-degree unit instead...
To keep the longer-duration cam-lobe -(of the 28-degree unit) from allowing all the excess-ACjuice -(5 DC-pulses) to pass through the ign.coil, simply install a power-diode -(8-amp & 250-PeakInverseVolts -minimum), inline with the YELLOW-wirelead prior to it's connection to the ign.coil . _ Doing so will then cause the ign.coil to only be subjected to about half of the excess power-pulses, (which happens to be roughly the same amount of unnecessary power-juice as the ign.coil is normally subjected to, with the stock 18-degree advancer-unit - [that's at least 2 power-pulses]).
___ To confirm the advantages of the 28-degree advancer unit (in conjunction with the power-diode), for use on the models which weren't stock with battery-powered ignition,, you end-up with:
_ easier starting -(due to a less advanced 'pre-set' ignition-timing -[12 vs 22-degrees BTDC]), AND,
_ much less directly-wasted power-juice, which helps to reduce motor-heat! - (Due to the fact that the alt.stator will run cooler, and also because less fuel is burned -[since the engine won't have to work against as much resistance from the alternator!]!)
____ Got questions? _ Please ask!


DUKE-Cheers,
DCT-Bob