Coils

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tban52

tban52

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Hello all,

I am a newbie. This is my first post. I just purchased a 75 commando. It started and ran (although roughly) when purchased. It will not start now. This is not really a problem because this is a restoration project. My question is concerning the coils. Upon examination it looks to me as though there is one 6 volt coil, and one 12 volt. The marking on one is 117M6, and the other is 17M12. One ohms out as a 6volter and the other as a 12. The bike has a ballast resistor that ohms out correctly, so I assume I should have two 6 volt coils. This has a points ignition by the way. If this bike did indeed have two different coils would it ever have run?
Which coil should I run? My intention is to install an electronic ignition. Can anyone help me with this?

Thanks, Terry
 
Welcome to the forum, Terry.

Yes, your Commando should have two 6V (Lucas 17M6) coils and a ballast resistor for the original points system.

Most electronic ignitions also use twin 6V coils (wired in series), or you could use a single 12V dual output coil instead?
 
Welcome aboard Terry

The standard setup is two 6V coils in series.
Not sure whether it would have run on that mixture of coils, were they wired in series or parallel?

If you're planning to go electronic the Boyer, Pazon and Trispark are the main players. Trispark have the neatest installation.
Some of the electronic ignitions use their own coils so may be best to decide which way you are going to go with ignition before spending money on a second 6V coil.
 
Rich_j said:
The standard setup is two 6V coils in series.
Click to expand...

NO, the coils are wired in parallel (to each other) for original points ignition.

Although you could say they are both wired in series with the ballast resistor, if that's what you meant?
 
The bike has a ballast resistor that ohms out correctly, so I assume I should have two 6 volt coils.
Click to expand...
Correct. Stock wiring used 2 6-volt coils and a balast. Only one set of points closes at a time, so effectively it's one 6 v coil and the ballast resistor each time.

If this bike did indeed have two different coils would it ever have run?
Click to expand...

The 12V coil side would have greatly reduced spark. It may have run, but probably poorly.

Which coil should I run? My intention is to install an electronic ignition.
Click to expand...

Electronic ignitions for Brit twins use a wasted spark configuration - both cylinders fire together. The choices are either 2 6V coils in series (most common) or one 12V coil with 2 plug wires (ala Harley). Neither configuration uses a ballast resistor.
 
If you are upgrading the ignition you might consider getting a dual-output Dyna coil to replace the ones you have.
 
tban52 said:
Hello all,

I am a newbie. This is my first post. I just purchased a 75 commando. It started and ran (although roughly) when purchased. It will not start now. This is not really a problem because this is a restoration project. My question is concerning the coils. Upon examination it looks to me as though there is one 6 volt coil, and one 12 volt. The marking on one is 117M6, and the other is 17M12. One ohms out as a 6volter and the other as a 12. The bike has a ballast resistor that ohms out correctly, so I assume I should have two 6 volt coils. This has a points ignition by the way. If this bike did indeed have two different coils would it ever have run?
Which coil should I run? My intention is to install an electronic ignition. Can anyone help me with this?

Thanks, Terry
Click to expand...

You could have the 12 volt coil not connected to the ballast and the other using it, compare what you have to the ones posted here wiring-diagrams-t4725.html The best is an electronic ignition with a dual output coil. Do a search on electronic ignitions, lots of information on the different ones available for Nortons.

Jean
 
Jeandr said:
You could have the 12 volt coil not connected to the ballast and the other using it
Click to expand...

As it is a "75 Commando" presumably it's an 850 MkIII model? So it really needs two 6V coils with the ballast resistor bypass wire from the starter solenoid fitted with points ignition. If one coil is 12V then that coil's spark will be weaker during electric starting.
 
L.A.B. said:
Rich_j said:
The standard setup is two 6V coils in series.
Click to expand...

NO, the coils are wired in parallel (to each other) for original points ignition.

Although you could say they are both wired in series with the ballast resistor, if that's what you meant?
Click to expand...


OK, from 1971 they were 6v coils wired in wired in series also in series with the ballast resistor, prior to the coils were in parallel . Since the original post was about 75 bike I stand by my statement.
 
L.A.B. said:
Rich_j said:
OK, from 1971 they were 6v coils wired in wired in series
Click to expand...

I can't agree. Check any '71-on wiring diagram, the ignition feed wire (White/Blue* - Ballast - White/Purple) goes to both coil negative terminals, therefore they are wired in parallel: http://rocbo.lautre.net/technique/norto ... p/150.html

*(White/Yellow = MkIII)


Rich_j said:
also in series with the ballast resistor,
Click to expand...

Yes.
Click to expand...

Whoops, you are absolutely correct. I was absolutely convinced they were series but just checked the diagrams and you are right.

Apologies for spreading misinformation.
 
Not to step on the OP's problem, but what are the options for a dual output 12v coil? I have heard of doing this with a Harley coil, but which one? or does it matter? Is the single coil set-up more reliable, more powerful, more better?
 
Single coils with two outputs are in fact two six volt coils pre-wired as you would for E.I. ignitions. This also means that one plug will fire from the center to the anvil and one will fire from the anvil to the center. On our bikes this arrangement is best when the OHM reading is somewhat high 3 OHM's or better. High output coils for HD's are a bad choice if they have 2 OHM's or less. The coils get hot and it puts a lot of strain on the E.I.'s black box as well. Read a lot before you jump into this.
 
There are several coil makers who build for Harleys (Andrews, Dyna, Accel, etc.) and looking at the published output, they are all about the same performance wise. The important thing is to look at the ohm requirement/limit of the ignition that you are using, and match the coil to that. Generally coil manufacturers make several resistance levels of of a given coil ( say 2 ohm and 5 ohm) to allow for the change by Harley to electronic ignition at some point. As I recall, Boyer specifies a coil with no less then 3.5 ohms, so you must pick a coil with 4 -5 ohms.
I have researched dual lead coils a bit and believe they offer more voltage to fire a larger gap and thus better performance, however I can't give you any specific data/examples of that. I plan to use one myself though. :wink:
 
It is impossible to assume the wiring harness has not been tampered with. It is hard to beleive with a 35 year old bike that the wiring is stock, even harder to beleive when it is obvious the coils are not the originals so unless the circuit is traced, no diagnosis is possible.

Coils have changed through the years, the old coils did not produce as much voltage because the turn ratio from the primary to the secondary was not as high back then, so even with the same primary resistance, the output of a modern coil can be a lot higher. Dual output coils are not wired differently on the primary side, only the secondary is different so instead of having a connection to ground, the two ends of the output coil are brought out, usually those coils produce a lot higher voltage than single output coils for the only reason that it has to jump two spark plug gaps. And don't think for an instant that a coil is good just because it can make a spark in open air, as pressure increases, a higher voltage is needed to jump the gap.

Jean
 
Jeandr said:
Coils have changed through the years, the old coils did not produce as much voltage because the turn ratio from the primary to the secondary was not as high back then, so even with the same primary resistance, the output of a modern coil can be a lot higher.
Jean
Click to expand...

Umm.. it's not turns ratio. Advances in coil design have centered on winding technology to reduce distributed capacitance in the secondary and improvements in magnetic materials for more efficient energy transfer. Unfortunately these parameters are never specified, and most people wouldn't know what they meant anyway.
 
I use a 5 ohm Dyna dual output coil with a MkIII Boyer ignition. Works great. I didn't run the bike much with the stock coils and have made a lot of changes to the bike so I can't comment on any performance gains with this coil relative to the original 6V coils. My bike starts consistently first or second kick and has a strong spark with this setup. The Dyna bolts right on to the original coil bracket with minimal modification. It does change the appearance of the bike so you need to consider that if you value the stock appearance with coils sticking out from the underside of the tank. I bought my coil from Old Britts.

Coils
 
I run CNW's single Crane coil conversion with a Tri-Spark ignition with a .035" plug gap and it's as good as it gets I think. Trouble and maintenance free. The coil conversion comes with a bracket and stainless cap screws. Very nice.
 
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