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So grabbed a new NGK CR2 spark plug wire assembly due to its copper core construction and find out it’s equipped with a resistor cap - 5000 ohms. The new NGK B7HS plug has a built in resistor value of 1000 ohms. All of this based on what I’ve read is just a product of modern design to deal with modern issues not of concern for bikes of our vintage. So here’s the question - given the already low electrical power output of my ‘73 350(converted to 12v) will the extra 6k of resistance have a bearing on performance? I plan to get rid of the boot resistor at the very least - figure it will contribute to a much healthier spark. Be interested to hear if others have addressed this already. Thanks guys

Hans?

Resistor plugs are mainly to reduce electromagnetic interference which might affect other electronics in a vehicle. Also, I remember CRT televisions having the picture disrupted by vehicles passing by. A side effect is to reduce the energy at the plug gap, although by how much I can't say.

Interesting, I thought the BxHS series of plug had no resistor whereas the BRxHS series does.

I had a few new and secondhand plugs around a while ago, and I measured them. About half had a resistance reading or were open circuit when there should have been no resistance. The resistor plugs were generally OK.

The same with plug caps - several were open circuit instead of short circuit (between the lead and the plug connector).

For the plug, the connection between the lead terminal and the centre electrode is made using a spring - not the most reliable electrical connection. In real terms, however, poor contact would be easily overcome by the voltage produced by the coil.

I x-rayed a resistor plug cap and a condensor that had no capacitance, and they too rely on press fit of parts to make contact.





(Resistor plug cap)



Unfortunately this is a subject like oil and tyres where everyone has a different experience - you'll get many opinions on this

As far as I know all NGK spark plugs with the letter R in the code have a resistor in the plug construction, ref. the plug caps I always use a non resistor type (not sure where in the world you are?) but in the UK the best non resister plug caps were made by 'Lodge' and were used widely on competition bikes......I think you can get lodge lookalikes on ebay which should be both cheap and resistor free.
Cheers,
George

Duccout wrote:Hans?

Aye!

But before giving any comment, I need to know if Mark3Cam's '73 model in question has a points ignition or an electronic CDI?

Hans

Good old points - but shiny new ones along with the condenser of course. From what Iveread it seems guys lean towards avoiding resistors with a points system and keeping them for CDI

Also, I remember CRT televisions having the picture disrupted by vehicles passing by

In the early 80's a neighbour complained that my 900ss was "so loud that it was interfering with her television"!

OK, std. points/coil ignition, despite a '73 w/c model should have a CDI (Motoplat or Ducati Elettronica) ex-factory. If you run a fully charged battery, this would be the better choice in terms of spark quality, because all components can be checked and exchanged in case of wear or damage.
About the pros and cons of the resistive elements in the HT current path of an ignition, I recommend to read https://motorcycleproject.com/text/plugwiretech.html. It is much more elaborate than any explanation I could give here in fewer words.
My summary: if all components of your ignition system ar in a good shape, a resistor either in the spark plug or the socket does not degrade the spark noticably. But, one must avoid a combination of both, this may decrease the spark current more than tolerable.
The important physical value for the spark quality is it's electric current. This in turn is limited by any resistance in the secondary coil circuit, i. e. the resistance of the secondary winding a n d any RFI suppressing resistive element. The coil's resistance is given by the manufacturer (afaik 2 ... 10 kOhms), is fixed, but plug, socket and the HT cable can have different resistances. On some vehicles i.e. cars, you find resistive cables made from some carbonized polymer fibers, not from copper litz wire as we know it.
Btw, the website linked above has some more very instructive articles available, like https://www.motorcycleproject.com/text/ignition_coils.html

Addendum 1: a good and inexpensive means to improve the spark quality of a points ignition is a transistor amplifier, replacing the points high current switch function: https://gammatronixltd.com/epages/bae94c71-c5b6-4572-89a1-e89006e78fbe.sf/en_GB/?ObjectPath=/Shops/bae94c71-c5b6-4572-89a1-e89006e78fbe/Categories/Category3 Simply put it between the points and the HT coil's primary. The points subsequently carry a very low current only and will last much longer. Merely, the wear of the follower block will remain and has to be checked/corrected as before.
I successfully installed them in the bikes of 2 fellows. Starting and idling of their motors were explicitly improved.

Addendum 2: such an amplifier makes the condenser/capacitor obsolete - one component less to fail !

Hans

r6ymy wrote:

Also, I remember CRT televisions having the picture disrupted by vehicles passing by

In the early 80's a neighbour complained that my 900ss was "so loud that it was interfering with her television"!

In the late 70s pretty much everyone in my street made comments to similar effect about my 900ss.