Magneto polarity

[greg brillus]

The comment I made earlier re the testing of the output on the test bench, and having 2 totally different readings, that is 200 volts with the polarity correct and around 120 volts with incorrect polarity taken at the primary can be duplicated on a running machine. I did so on my Comet years ago.....remove the mag cowl, and the black end cap off the magneto. Start the engine (may need an assistant to help here) and using a volt meter, place the black test probe on the magneto housing, and hold the other red one on the rotating center bolt that holds the points assembly in place. Much safer than testing the HT output side of things......unless you want to look like a human flash cube or similar. Magnetoman may be able to elaborate a bit more on this, but it does work.......Cheers..........Greg.

 

[Ken Targett]

Must admit, Greg, I don't understand why you're getting those results. Indeed, if by "correct polarity" you mean negative, I'd have thought that the peak LT voltage would be lower than with incorrect/positive polarity.

Assuming it's true that with negative polarity HT output the spark is produced in a hot engine with traditionally shaped plug electrodes more easily than with positive, then the HT voltage won't have risen as high with negative as with positive when the spark is struck. Immediately the spark commences, the load it places on the magneto drags the HT voltage right down. The lower peak HT voltage with negative compared to positive will, I'd have thought, be reflected as a lower peak LT voltage with negative compared to positive.

I'm wondering what sort of peak voltage meter you are using, and whether it works accurately with the very brief pulses that we're talking about, and just as accurately for either polarity. I did have a go a few years ago at making a peak voltage meter to measure these things, but abandoned the project because I was getting spurious results.

 

[greg brillus]

Sorry Ken, It's been a while since I did this, so I can't remember which lead red or black equates to the polarity being negative or positive, but the results were pretty much always the same. Yes the tests were carried out with what Ivan said was an analog peak voltage meter, as he said a normal multi meter would not work properly. He also told me that the best way to test it was with an oscilloscope which we did not have, and I would say almost 100% of most Vincent owners would be the same.....Methinks. There was definitely a common difference of around 80 volts at the primary, at a given RPM. Cheers.........Greg.

 

[Magnetoman]

Thanks mate, much appreciated.

You're more than welcome. There is a lot of misunderstanding and misinformation about magnetos and I enjoy using my knowledge of them to help. The magneto restoration thread I linked to in my previous post has received over 30,000 views in only ~18 months, which also has resulted in quite a few people contacting me directly out of desperation after failing to get theirs fixed. One of the more interesting contacts was a month or two ago when I got an email in French. Although I've studied French I speak it badly, and I speak the arcane Magneto French dialect not at all. It took a number of exchanges mediated by Google Translate (which isn't much better than me at Magneto French) for him to communicate the problem and me the solution, but he was thrilled to get his magneto working.

That's not to say my efforts are universally appreciated. I've given a number of workshops on magnetos at rallies I've attended in the U.S. and Europe and at one of them someone in the audience was dismissive of anyone wasting time repairing old junk electrical equipment when modern replacements were much better. He was quite insistent, and I remember one of his arguments equated used an old magneto with using old tires. I learned later that evening from someone else in the audience that the guy sold replacement magnetos. So, not everyone is thrilled that I share what I know about them. But, hey, you can't make everyone happy, can you?

 

[greg brillus]

I think the thing to remember is that, with magnetos...they are a bit of an unknown to most folk, even though they are a fairly simple device, and been around for a very long time. So although the info that you Magnetoman, and Ken provide is brilliant, I think the average Joe probably struggles to understand it. When all things modern lead us away from the conventional, and towards electronic/ battery ignition. It too has its issues and problems.......this is one of the reasons I have chosen to use a brand new Joe Hunt magneto on the Vincent twin roadracer I am building at the moment.......No batteries/coils and electronic external boxes for me......I have full confidence in Magnetos........look at how many of them are flying over your heads...!!. Cheers......Greg.

 

[Magnetoman]

the thing to remember is that, with magnetos...they are a bit of an unknown to most folk, ... [and] I think the average Joe probably struggles to understand it.

Yes, that is true. However, my career in real life requires me to patiently explain quite complex principles of physics to students, some of whom are not nearly as eager to learn as one might hope. So, since I spent a decade unraveling the mysteries of magnetos at the most fundamental level of every one of the phenomena involved, I'm more than happy to do my best to transmit that knowledge in as understandable a fashion as possible. Whenever I fail at this, by all means ask me for clarification.

Nearly a century ago (1918) A.P. Young wrote, "The magneto: he who holds this key controls huge industries essential to war and peace." My goal is more modest, helping people understand their motorcycles' magnetos in order to properly rebuild them (or to find someone with the equipment and knowledge necessary to properly rebuild them for you). As you also rightly noted in your post, there is no reason why the magneto cannot be one of the most reliable components on a Vincent.

 

[Kansas Bad Man]

Things in the day were much much simpler. My 49 ford had a duel point distributer and duel Mallory condensers, couldn't buy a better
street racing ignition .

 

[redbloke1956]

Sorry Ken, It's been a while since I did this, so I can't remember which lead red or black equates to the polarity being negative or positive, but the results were pretty much always the same. Yes the tests were carried out with what Ivan said was an analog peak voltage meter, as he said a normal multi meter would not work properly. He also told me that the best way to test it was with an oscilloscope which we did not have, and I would say almost 100% of most Vincent owners would be the same.....Methinks. There was definitely a common difference of around 80 volts at the primary, at a given RPM. Cheers.........Greg.

Hi Greg, I have a dual beam CRO if you ever need one for further testing.

Kev