Spark Plug Wires and magnetos

In other posts I have seen the warning against using carbon core wires with a magneto.
I have an Allis B with copper core wires and an FMJ mag. I also have a WC with carbon core and an FMJ mag. Both tractors are hand start and will start easily when I put enough fuel in them. I usually only run them for about an hour once a year.

1. Am I damaging the WC's mag with the carbon wires?

2. Why are carbon core wires bad for magnetos (technical explanation please)?

Okay, heres my oldddddddddd Electrical Engineers take on your question, but mind you Im rusty as an old nail especially in areas other then AC Power Distribution where I practiced in my later years... Lawyers love disclaimers lol... Hope Professor Jim and Bob and other more current and knowledgeable electrical minded sparkies can weigh in here (but lay person Billy Bobs are sure welcome also, were never too old to learn I figure), I may be all wet, it wont be the first time grrrrrrrrrrrrrrrrr


"1. Am I damaging the WC's mag with the carbon wires?"

I wouldnt say your mag is gonna crash n burn due to carbon wire use, only that it may be working a bit harder (maybe running just a tad hotter and less efficient) and when the coils stored energy is dissipated youre expending a part of that finite energy as wasted I Squared R heat energy in those resisitve plug wires versus across the plugs gap which ignites the fuel. Its the sudden release of energy and heat when the arc occures across the plugs gap that ignites the fuel, why release any of that limited energy in the resistive wires????

2. Why are carbon core wires bad for magnetos (technical explanation please)?

Pretty much same as above, the mag was designed so the energy stored in the coil was to be discharged n dissipated across the plugs gap and the impedance into which the HV secondary is looking was originally very low (if steel or copper plug wires were used) other then the open gaps (until she arcs current), of course. Once the plug gap arc occurs and current begins to flow, if the wires are resistive that reduces current flow and wastes energy in the form of I Squared R Heat ID PREFER IT ALL BE DISSIPATED ACROSS THE PLUGS GAP NOT IN THE RESISTIVE WIRES

In summary, far as I know the mag was designed to be looking into a lower impedance (steel or copper wires) then that presented by resistive wires and its my old rusty engineers opinion the mag will run a tad hotter and be less efficient if resistive plug wires are used and I know FOR SURE sure any heat energy expended (I Squared R) in the resistive wires reduces that which would otherwise get expended across the plugs arc.

So until proven otherwise, Thats my story n Ima stickin to it lol

Ol John T and all Lookin forward to other informed opinions and info God Bless yall

IMHO, you got an ELOQUENTLY written and thoughtful response from JohnT!

I will add to the discussion that the function of the carbon resistor wire is 2 fold. TO increase the spark voltage a little (9000ohms per foot of wire), which makes a little more energetic spark at the gap, and to prevent the wire from being an antenna for radio waves. THIS is more of a suppression by conductor wave propagation than a absorption of energy. The resistive element mentioned by John is sure involved, but not for suppression of RF. Magnetic suppression wires are real copper or stainless wire wrapped around a stranded Kevlar or glass fiber core. These have the near zero resistance of a solid wire (they are a continuous conductor) and the winding of the wire in a coil suppresses the RF noise.
A single (to me most important) issue with carbon wires is that they deteriorate and can/do become near non conductors in a way that can put far more resistance into the circuit than 10 000 ohms or so. 40 000 ohms can cause coil failure and cap/rotor failure from no place to send the spark.
as it fails, the tractor still runs well, but the mag (or coil for that matter) is being stressed.
Neither Magnetic, nor solid wire have that issue if kept in good condition. Jim

Interesting info Professor,,,,,I have seen both the wire itself and the illustrations of the helical/spiral wound wire used to reduce RF interference, they look expensive but dont have the high carbon resistance of just cheaper carbon core wires. AS the plug gap widens and/or resistance increases it takes a higher voltage to arc jump the plug gap but theres always engineering trade offs. Theres only finite X amount of energy stored in the coil during the current conduction phase which gets expended into heat losses and V x I x Time when the field collapses to raise the Secondary voltage to fire the plug. Therefore, if you widen the gap (or I guess add resistance) which necessarily increases the voltage rise before current arc jumps the plug gap, either the current and/or the arc duration time must decrease.

Looks like all agree carbon core wires are NOT favored for old tractor magneto ignitions but the theory gets a bit complex, maybe more then I spoke of......

John T

Thanks, where shall I send your check Bob lol

Actually, I think its a bit more complex but Im too tired n old n lazy to research it

John T

Hey John T - You pretty much said it all!

About the only addition I can make is RFI wires are unnecessary unless you are intent on listening to an AM radio while using your tractor. IE use solid copper wires with your magneto - it is what the engineers who designed the ignition system intended!

...Bob M

Sounds like between the responses I need to add change spark plug wires to my summer pre-start fix it list for my WC. Thank you both Janicholson and John T.

You are welcome. Jim

The Sun Electric class that I took on their Auto Diagnostic Equipment had us testing spark plug wires for resistance. Anything over 1000 ohms per inch or over 10,000 ohms maximum per wire as reason for replacing them.

Speaking as an electrician, and an engineer(NOT electrical), but as an experienced tractor engine cranker since about 1953, I can say that carbon filament wires have resistance which reduces the energy available at the spark plugs. Since more energy results in quicker starts and less cranking, replace the wires with solid wire core ones. Lots easier on your cranking arm, especially in the winter.

Carbon powder can get out of the ends of the fiberglass threads.After enough is gone an arc will start opening a gap in the wire.I have found many wires that read open.Test sets have a wire test function.The reading can predict that a wire is failing It has been considered bad practice to let a magneto fire into an open circuit because the secondary winding can arc to it self.You can hear a fence charger secondary arc to it self.A normal fence line loads the charger enough to keep the voltage peaks down.Fence chargers are run after repair with a 500 ohm load for testing.The Model T coil I use for testing has its own spark gap.No engineering but I have three 5 gallon pails of bad fence charger transformers in the shop now.All have been damaged by lightning or running on open circuits.New chargers are starting to have spark gaps in the case.No reason to have fake wire on tractor ignitions.