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Magnetos, Ignition Coils and Spark Plugs Discussion about magnetos, buzz coils, spark plugs, ignitors and low tension coils.

Magnetos, Ignition Coils and Spark Plugs

H-D magneto (Harley Davidson)


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  #1  
Old 03-13-2008, 07:34:50 PM
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Default H-D magneto (Harley Davidson)

Hello all,

Helping a friend with his latest find. A 1960 Harley Sportster with the magneto ignition. What an odd duck. Normally you would expect a twin magneto to fire every 180 degrees. Since this is a 45 degree V twin number one fires then the other fires 22.5 degrees short of 180 degrees later. Then of course the other fires 180 plus 22.5 later. The way I normally time mags is with an o-scope. I adjust the points to open at the peak of the coil voltage curve. This gives the hottest spark and works quite well. On this mag the coil pulses are spaced 180 degrees apart but the point opening events are not. This means they cannot both line up at the peaks. Either one spark is really great and the other weak or the other way around. I ended up splitting the difference and the spark is pretty good on both. I really can't think of any practical way to build the magneto to allow for this odd timing arrangement.

keithw
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Old 03-14-2008, 03:38:16 AM
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Default Re: H-D magneto


this may help explain it took me ages to work out how the system worked
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Old 03-14-2008, 09:59:20 AM
Stephen Girouard Stephen Girouard is offline
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Default Re: H-D magneto (Harley Davidson)

i'm not sure about the mag. model but on all distributor models both cylinders fire at the same time, one just inefectively on the exhaust stroke. the other at its correct fireing angle.. and the two were afew degrees apart, on the lobe which opened the points one lobe was wider (longer duration) than the other. One lobe for front cylinder ,one lobe for the back , each fireing both cylinders at the same time. There were other engine manufacturers who also did this with twins , but at the angle of the cylinders on a harley this fireing ofset had to be done .
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Old 03-14-2008, 10:27:23 AM
Stephen Girouard Stephen Girouard is offline
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Default Re: H-D magneto (Harley Davidson)

it will start & run on 1 cylinder if its timed 180degrees out, but will be underpowered & run on only 1 cylinder. i found this out at 19yrs old with a panhead, it took me and a old tractor machanic a whole day to figure that out. he had some choice words for a harley that day
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Old 03-22-2008, 09:48:48 AM
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Default Re: H-D magneto (Harley Davidson)

We built a test stand with my Ajax electric starter to save three old men from kicking this pig (or is it hog?). Fired right up but ran only on the front cylinder. After lots of hair pulling pulled the mag and went back to the scope. Since the point opening events are not 180 degrees apart and the magneto coil pulses are, I split the differance and put one on the leading side of the pulse and the other on the trailing side. Big mistake. Not enough energy has been built up in the coil on the leading edge. Set one opening at the peak of the pulse and the other on the trailing side. Makes good sparks and runs on both cylinders. With the point gap set at the spec both openings are on the trailing side. With the points closed the coil is storing energy for a short time so it is much more tolerant of late opening than early. This stuff just keeps teaching me new things.

keithw
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Old 03-25-2008, 10:38:16 AM
JoeCB JoeCB is offline
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Default Re: H-D magneto (Harley Davidson)

Kieth W , you mentioned that you can set up a mag for peek output by using an o-scope. how is this done? Are you hooking the scope to the primary ( low Volt ) side and looking for the volt peek? can you help w/ the procedure?
Thanks

Joe B
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Old 03-25-2008, 05:16:48 PM
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Photo Re: H-D magneto (Harley Davidson)

http://members.aol.com/keith0alan/images/magneto.jpg

Start by figuring out the proper direction to turn the mag and hooking up a battery drill or something to turn it. Unhook the coil primary from the points. The primary lead feeds channel one on the scope. Leave the secondary lead unhooked. It will generate enough voltage to get your attention but not enough to hurt it's self. Hook a 10k ohm resistor to the points and a 12v battery. When the points are open they will have 12v on them and when they are closed there will be zero volts. This goes to channel two on the scope. The resistor and battery are required to provide a way for the scope to see the points opening and closing.

When you spin the magneto channel one will show a posative and a negative pulse each revolution. Channel two will show when the points open and close. You normally want the points to open when the coil voltage is at it's maximum. On my Ajax mag (American Bosch) that was at the peak of the negative pulse, the posative pulse was not used.

If you can't align the points opening with the peak then a little late is better than a little early. With the points closed and the magnetic field collapsing energy is stored for a short time.

On the Harley (a F-M magneto) with the points set at factory specs the points openings happened after the peak on both pulses. I adjusted them so they opened at the peak of one and after the peak of the other. (non-semetrical firing v twin). On an engine that fires evenly like a vertical twin the point openings can be aligned with both pulse peaks.

Let me know if I missed anything.

keithw
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Old 03-25-2008, 07:27:21 PM
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Thumbs up Re: H-D magneto (Harley Davidson)

Thanks, Kieth
Makes sense, The scope will show the secondary peek and the points opening. Next time I'm messing arount with one of my motors I'll try the O- scope.
Joe B
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Old 03-25-2008, 08:22:07 PM
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Default Re: H-D magneto (Harley Davidson)

You're looking for the primary peak with the scope. What goes on in the secondary is real magic. Can you say "shock excited LC resonant circuit". If you want I can explain it but it takes a little while.

keithw
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Old 03-26-2008, 09:50:54 AM
JoeCB JoeCB is offline
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Default Re: H-D magneto (Harley Davidson)

Kieth... Sure, let's go for it! The mysterious process that produce that 15 K V from out of "nowhere" has always intriguied me.
Joe B
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Old 03-26-2008, 10:37:06 AM
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Default Re: H-D magneto (Harley Davidson)

O.K., I'll try to get it correct.

Points closed, rotor spinning, coil and condensor are shorted to ground. As the magneto starts to generate current a magnetic field builds up around the iron core the windings are wrapped around. At the peak of the current pulse the magnetic field is at it's greatest. Points open. The magnetic field begins to collapse. At this point the coil and condensor form a series resonant circuit. Since the condensor is a fairly high impedance load the coil magnetic field collapses really fast. This counter EMF induces a really high voltage in the coil primary. In the order of a couple hundred volts. This voltage charges the capacitor. Once the magnetic field has collapsed all the way it generates no more voltage. However the electrical energy is stored in the condensor. The electrical energy in the condensor discharges through the coil. This creates a magnetic field, storing the energy. The energy transfers back and forth between magnetic in the coil to electrical in the condensor. The frequency that this happens is the resonant frequency of the circuit. High kilohertz to low megahertz range. As there are losses it only bounces back and forth a dozen times or so loosing energy each time. This is called a "damped sine wave". Early spark gap transmitters worked this way. So where does the spark voltage come from? While the energy is bouncing back and forth the magnetic field is also cutting the wires in the secondary windings. Since there are 100 times more windings there the voltage is stepped up from about 200 to about 20,000. In the process the secondary doesn't produce just one spark but rather a series of sparks as the energy bounces back and forth between the coil and condensor. If the condensor is open only one spark weak will be created. The purpose of the condensor is primarily to create the LC resonant circuit. Keeping the points from burning is a secondary benefit.

Spark? It's not a spark, it's an radio frequency plasma discharge! The old style low tension ignitors make sparks, a single collapse of the magnetic field producing one pulse of several hundred volts.

I think that pretty well covers it.

keithw
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  #12  
Old 03-26-2008, 11:48:09 AM
laren Allred laren Allred is offline
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Default Re: H-D magneto (Harley Davidson)

Thanks Keith. You nailed a complex event very well.
In magneto talk the point of rotation where the maximum lines of magnetic force are cutting the max number of windings in the primary coil is referred to as the E gap of the magneto. This is the period of rotation where the points should just open. and is usually independent of the point spacing.( sometimes referred to as internal mag. timing.) This E gap is marked on some magnetos but can be determined with a scope as Keith advised. For max. output adj. the points to open at this point and then set the timing by rotating the position of the Mag.
As per the uneven firing of Harley and most other narrow angle v twins, there isn't much we can do about it. other than tweek the internal timing of the mag and hope to reach a compromise that will make both cyls. happy. This is actually an engineering / design problem and would need two separate ign. systems to solve.
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Old 03-26-2008, 12:20:51 PM
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Default Re: H-D magneto (Harley Davidson)

I had read many automotives books but never got a good explination of the circuit till I stumbled across it when I was studying radar circuits.

keithw
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Old 03-27-2008, 07:04:09 PM
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Thumbs up Re: H-D magneto (Harley Davidson)

Thanks Kieth, That was enlightening. I didn't realize that the secondary output was multiple discharges and how the capacitor facilitates this. Which brings up another ?, with reference to small engines; one lungers / outboards etc. ... just how important is having the correct size (capacitence) condenser? many times the mfd rating is not specified or available on older motors.

Joe B
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Old 03-28-2008, 11:02:08 AM
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Default Re: H-D magneto (Harley Davidson)

In theory with the correct size cap the points don't wear. The wrong size and material gets transferd from one point to the other. The little peak and pit you see on points. Too much capacitance and the peak is on one side to little and it is on the other. I don't recall which way and I haven't seen an explination as to why.

keith
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Old 03-29-2008, 12:57:12 AM
bill chasser bill chasser is offline
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Default Re: H-D magneto (Harley Davidson)

Have you got the sporty running yet? Timing is important with these as you dont want the engine to sieze and it will if you mess around with this too much and dont keep a handle on combustion temps.

What model are we talking about ie K or XL mdls?

As a person who owns a very hot ironhead sporty let me add a few things from personal experience. I dont run the FM or cirrillo mags as my engine is a later design. But given that this unit is a recent purchase I would do several things to the eng before you go any further. 1) remove the intake manifold from the cylinders and if the seals are of the o-ring type throw them away. Buy the rubber band style intake seals and get the aircraft quality wide stainless clamps. Before you reinstall butt the intake up against the cylinders and check to insure that the end surfaces of the manifold and cylinders mate up flush. If they do then you are in good shape. If not then you will need to get a fresh set of head gaskets to reposition the heads to mate the manifold correctly. If you do not do this you are setting yourself up for a siezure from air leaking through the seals and leaning out one or both cylinders under normal loads. I am using a S&S Two-throat but yours most likely will have a bendix or tillotson if originally equipped.

I have a H-D sporty manual that Im sure has mag info in it but it is not imediately accessable. I will chase it down if you need any additional info. What mdl FM do you have? Do you know it to be correct for H-D and not of some cat starter motor? Hey stranger things have happened especially since you are a new owner and probably bought this as a basket case.

You can PM me if you need info or write here for everyone else To overhaul a FM runs about $300 out here in CA from Hunt Magneto who does all my other mag work. Make sure there are no arc trails on your caps underside and that you are working with a fresh set of platinum plugs. Im not kidding make this easy on yourself in the short run it will be worth the effort.

Im not particularly familiar with the timing marks on your crankshaft as they vary from yr to yr. You should be able to static time it and be in the ball park. It will be timed to the front cylinder.

Have you had the cam cover off?

Well I just dug out my H-D book and it goes only back to the external distributors of 1970. Mine is a 1974 with an electronic race module which doesnt help you. I can get access to mag info from a friend but will be taylored to the FL mdls but should parrallel what would be needed. let me know...

Bill
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Old 03-29-2008, 09:11:09 AM
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Default Re: H-D magneto (Harley Davidson)

Thanks for the info, I'll pass it along. It's a 1960 XLCH ironhead. It is the original single bore carb. It is a correct harley mag with the two different size lobes. We've pretty well gotten the mag sorted but it does need some internal bits. The top bearing and point holder plate is broken up and has been glued back together. My thought is that someone either dropped it on the drive gear or tried to remove the rotor without removing the plate first. Anyway if you know of a source for internal bits I would love to hear about it. The bike belongs to a friend of mine. Another friend and I are helping with it. As I have space it is here. Having the test stand with the external electric starter has allowed us to make as much progress as we have. We have been able to kick start it after a little warm up on the stand but it's not quite sorted well enough to kick start cold. We're getting there. I expect it will be like my old Triumph, everything has to be just right or it's just an exersize machine.

One big question, It has an oil pressure gauge on one of the rocker boxes. Do you know what kind of oil pressure we should be seeing there?

If you've got any info on the F-M mags I would like a copy if possible. Good information is hard to find.

Thanks,

keithw
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Old 03-29-2008, 03:23:10 PM
Stephen Girouard Stephen Girouard is offline
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Default Re: H-D magneto (Harley Davidson)

another thing i had to do on some for what its worth, when there was a little wear on the distributor ,or mag, bushings i had to adjust the points a little closer, they'd run high rpm and the points would open eratically, opening wider than they should up in the rpms,
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Old 03-29-2008, 03:24:56 PM
Stephen Girouard Stephen Girouard is offline
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Default Re: H-D magneto (Harley Davidson)

is your knee hurtin yet LOL
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Old 03-29-2008, 03:45:12 PM
bill chasser bill chasser is offline
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Default Re: H-D magneto (Harley Davidson)

Kieth

Parts for the FM mags are readily available from any magneto shop. The internals are all pretty standard pieces. Like I said I use Joe Hunt Magnetos here in The Sacramento are and are top notch rebuilders doing all type from Cats to top fuelers.

As for info it will take me a few days to track my source down for his FL manuals.

If this helps any inre to the 1970 distrubutor mdls it says that the narrow lobe times the front cylinder and the wide lobe times the rear.

So many of the sportsters have ben cobbled over the years with backyard hack jobs that some times you never know what you really have. Pull the Flywheel inspection plug and #1 plug to determine the timing marks on the crank. there will be tdc.bdc and 30degrees advance for #1 cyl. If im not mistaken the mag should be static set at tdc for #1 as the mag should have a n advance curve set upinto the unit itself but I will get the info for sure

Bill
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