Question about coils

The coil on my 1973, 750 went out.
It has a Boyer Brandson Mk III ignition.
Can I use a 5ohm coil instead?
If yes are there any negative "side effects" to doing so?
Have a Harly 5 ohm handy but do not want to burn anything up.

Thank you in advance.

Henry Bowman said:

Have a Harly 5 ohm handy but do not want to burn anything up.

Click to expand...

Is it a single or a dual coil?


"3) The maximum ignition coil current through the unit must not exceed 5 amps.
The total ignition coil resistance should not be lower than 3.5 Ohms."

Remember the original Norton coils are 6V.

It'll work with 12V coils and the bransden MkIII ignition. I used the big tube 12V coils with that ignition initially for a couple of decades and later used a very inexpensive 4.5 ohm dual coil with the bransden ignition without issue for the last 2 years on a 750. I got the coil from Dime City Cycles, but I see you are in Guam so that may not be a good source. They still sell that coil for $35.95US. Shipping might be a deal killer though. I want to say that 5 ohm Harley coil should work, but have never tried it.

Another quote from some Bransden docs: For low compression engines two 12 volt coils (4 Ohm) in series are satisfactory, for racing and high compression engines two 6 volt coils (2 Ohm) in series, or one 12 volt 3.5 ohm or more primary winding resistance dual output coil will give the best results.

I used this dual output coil with Boyer MK3 on the 920. The seller said it would work well with this ignition.
The bike starts easily and runs pretty strong, so I guess he was right about that!

What a coinkydink

That dual coil is the same one Dime City Cycles sells and the very one I'm using currently on my 750 engine. I used it on the Mk3 and Mk4 Bransden ignition products without issue. It is currently in use with the TriSpark ignition. The Boyer ignitions are boxed up on the shelf. That dual coil is being sold for as much at $100US by some vendors, but Dime City Cycles sells it for $34.95US.

12V 4.5 ohm dual coil at Dime City Cycles

Thank you!
I will order one, and it sounds like we can use the 5 ohm in the mean time while I wait for the new one to get in.

Again, thank you all very much

Henry Bowman said:

Thank you!
I will order one, and it sounds like we can use the 5 ohm in the mean time while I wait for the new one to get in.

Again, thank you all very much

Click to expand...

If talking about the 12V 4.5 ohm dual coil mentioned above, you have to make up the plug boots. The wire in the plug wires is stainless, so resistor plugs are a good choice. I'm not a fan of the hard resistor caps of old, but they probably also work with a standard plug.