Although I'm open for a possible effect on spark duration - from school I vaguely remember something about Rc time - I doubt a significant
benificial effect will be noticed on our bikes.
What I
do know, from experience, not only my own but also others' (multiple riders reports): the plugcap has always been the #1 troublemaker on CB500/550s, which for the rest were wonders of reliability. This
must have to do with where they are located: not only exposed to oncoming rain and dirt, but, enclosed by the nearby head, also vulnerable to arcing
Right now I'm working on the plugs and caps of mine, so I can share some findings that might interest you. In he past I've run stock coils with NGK 5kΩ plugcaps which seems almost common practice. In the 80s Honda The Netherlands had advised me this. Dyna coils and caps however forced me to fit resistor
plugs, as the Dyna caps do not have them.
The other day I bought 4 new NGK DR7EA plugs and I couldn't resist (no pun intended) measuring them. BTW I recommend to always do this, at the counter or - like I did - right after having left the shop.
If a plug shows
anything odd, at least there can be no discussion. The other two multimeters at home confirmed: 6,1kΩ, 6,2kΩ, 6,5kΩ, 6,6kΩ. Now these are new plugs. Intriguing is that my
used DR7EA plugs show much lower values: between 3 and 4kΩ. Where resistance in these NGKs has
decreased over time, it has
increased in the Denso X24ESR-Us in use. I used to run these in holidays (long distances at high speeds on German Autobahns). These Denso's were
new around 5kΩ. When I replaced them, they read: 6,9kΩ, 7,8kΩ, 7,9kΩ and one even 8,5kΩ!
All the above measuring was done on cold plugs. What the resistance will be in the furnace of the combustion chamber? I have no idea.
I have not been able to find any Honda document that is specific on resistance in the secundary circuit. One of the best CB500/550 manuals I have, is the 100 page French one which has been compiled in close cooperation with Honda France. It is also the only manual that mentions something about resistor caps (p.84).
En cas de remplacement, il est important de monter des antiparasites ayant une resistance inferieure à 8000Ω sinon il y a perte de puissance d'allumage.
In translation: over 8kΩ, no good.
I'm no expert, but I can calculate. Over 20kΩ in a secundary circuit is a lot. Therefore I personally will not risk reliabilty for a so far only
theoretical benificial effect on spark duration. Until proven otherwise, I recommend 5kΩ in either plug or cap, but
not in both. But I invite anyone to experiment.
As said, I'm no expert. Here's something I do not understand. If you run no resistance at all, how could that damage the coil?