I'd like to add what I wrote the other day in the thread 'New spark plug Caps'.
The 550s in the US all came with 10K caps. This makes the spark duration longer, and is beneficial for the fuel burn.
It's not the first time I read this here (also Hondaman claims this) and I wonder if there's any proof. I really would like to see it, the more - although it is a possibility - this 'tuning of the spark duration' is new to me and I have not seen it mentioned before anywhere else* (not that that means much).
What do
I know? Not much. I bought my bike with 33.000 kms on the odometer and when, at around 45.000 kms, I had a look at the plugcaps for the first time I noticed a difference in resistance. One cap that appeared very old (it could have been original) measured 10KΩ and the others varied from 5 - 8 KΩ. As I could not find any documentation that specified (and up to this day haven't found any), I called the importer, Honda The Netherlands. The gentleman who was there in 1981 to answer technical questions, said all caps should have the same resistance and advised the 5KΩ. This was also common practice at dealers (and they
had 10KΩ stocked as well, for Goldwings).
If I study the Parts List of the CB500/550K3-4, my impression is it had to do with local legislation. Another possibility is that Honda had learned that more and more people fitted radios in their windjammers just like Goldwing people already did in mass.
So what to choose? Now over time
all caps increase in resistance and if you start with 10KΩ caps, you're already much closer to the point you have to replace them. So if there
is any benefit, it will come at a price.
Some claim 10KΩ caps will result in a longer spark duration over 5KΩ, others say it's the exact opposite.
Anyway, I doubt if an effect, if there's any to speak of, will be noticed by us in daily use and I would not go so far to deduct it will result in a better fuel burn. I must confess that to me an advantage of 10KΩ over 5KΩ caps is counterintuitive. My gut feeling, which could be wrong, is that 5KΩ will
all in all offer more energyreserve, but even then, will I ever notice it? Remember that the sparkplug's only task is to ignite; the flamefront of the burning mixture does the rest. A comparison of scope images that show significant differences can prove my gut wrong. Now as this subject comes back and again, I'll see if I can contact some experts next week. Maybe they can enlighten me some more on possible effects on spark duration. If they'll find my question interesting enough to answer, I'll report back ofcourse.
The common denominator for the forum is the stock configured bike.
I studied some parts lists to see if I can detect what resistance our bikes originally came with. Maybe I found the key in p.11 and p.81 of the parts list for the CB550F2 models.(
http://www.honda4fun.com/pdf_parts_list/pdf_spac550/CB550F2/CB550F2_1.pdf -
http://www.honda4fun.com/pdf_parts_list/pdf_spac550/CB550F2/CB550F2_4.pdf) What we learn here is that
only the model for Canada had resistor plugs. Yet
all models (except for Europe btw) shared the same plug caps including the Canadian model. Now if those caps would have been 10KΩ, than the Canadian model
must have ended up with a total resistance of 10 + 5 = 15KΩ, which is a bit much IMO and I wonder if there would have been much spark left. Personally I feel it's safe to conclude those caps will not have been 10KΩ. My guess is - but it's but a guess - they will not have been much higher than around 7KΩ or it would have caused problems on the models that had the resistor
plugs as well. I also tried to figure it out in the CB550K3/K4 Parts List and my first impression is, it's the same situation: models
with resistor plugs (now not only Canada but also Europe) share the same plugcaps that markets that
didn't have resistor plugs, like the US, had. But I must confess that particular Parts List always gives me a headache, so maybe someone can check me. P 11, 12 and 92 in
http://www.honda4fun.com/pdf_parts_list/pdf_spac500-550/CB500-550_K3-K4.pdf*
I remember that I've tried to learn more on this more than a decade ago, but back then I couldn't find anything on 'tuning of the spark duration by adding resistance', not in some standard works on automotive electrical technique, nor in the Bosch Technical Curriculum that even dedicated a separate volume to Battery Ignition Systems. 
