About your last query: having one plug circuit with lower resistance and one with higher resistance simulates what happens when one of the caps burns out in these bikes. The lion's share of the current during coil discharge then routes to the lower-resistance side whiel the higher-resistance side gets a [slightly] later spark as the current builds up far enough to jump the plug's gap. In actual bikes, this makes one cylinder of the pair slightly pop (backfire, softly) during a long decel drift down a hill, the first clue that the plug involved is not burning as much fuel as its 'partner' on that coil. on a 4-pipe bike (like mine) it is pretty easy to diagnose which side, and I have done this drill 4 times in the 145,000 miles my bike has experienced so far. In each case the 'pop' would start to appear when the resistance of the plug caps varied from one another by about 800 ohms or more, but it still ran Ok. Changing the caps solved it each time.
On the 550, all but the 1st year (1975) used 10,000 ohm caps OEM. This happened when they switched the coils to the 4.3 ohm primary versions from the original 500/750 4.5 ohm primary versions: the 500-4 had the same coils as the 750, despite the middle partnumber being different until 1990-something when they "harmonized" some of the electrical parts' numbers. Even the mounting brackets were the same, but with different middle partnumbers, and interchanged just fine.
The 4.3 ohm primary coils make about 400-600 volts more spark voltage at 7500 ohm plug caps that do the 4.5 ohm coils. Into 10,000 ohm caps this voltage is slightly higher. The difference is due to the engine difference: the 750 head is a swirl-charge design until the -392- head, when it became a semi-hemi instead. That corresponded to the 750 then also receiving (...drum roll...) 4.3 ohm primary coils, later harmonized to become [...cymbal clash...] CB550 coil partnumbers with the extension -630 as the last 3 digits, for both bikes. When holding the coils in your hand, they have the same "TEC" maker's mark and part number on them... today, if you order the -300- partnumber coils for the 750 (with the -6xx last 3 digits) they are the identical parts as for the 550, as I have recently ordered both.
The reason the slightly-longer spark duration matters in the 750, but not in the 500/550, is: the 750 swirls the intake charge into what Honda once used to call the "whirlwind stratification" (which you will also see referred to in their articles on their racing engines of similar design during the late 1960s and early 1970s) while the 500/550 do not. The general idea behind it is: while the spark is happening, it is igniting the fuel mix that is spinning past the spark. In swirl-charge engines this translates to a wider flamefront for larger ignition: in the undersquare 750 (and 350F) this also means it pushes longer down the power stroke than if not swirled. In 750 engines that were hemi'd (like the one in my book) this proved out: the low-end torque increases while the top speed decreases. Thus, it is useful to hemi the 750 head with big bores (beyond 63mm), but not below that, all else being equal.
In the simpler 500/550 chamber without swirl and an oversquare stroke, and more overlap ignition sensitivity (aka weak low-end torque) to long-duration sparks, a shorter spark duration matters. In the later 500 with the 10k caps, this also comes slightly later in time (as measured in microseconds) and helps reduce the backburn issues that plague this engine and caused Honda to spend a lot of Yen to make long intake runners to hold the carbs - just like on the singles of similar design. Mechanically delaying the spark advance improves both the low-end torque and cleans the plugs, letting you run colder plugs on hiway trips at today's Interstate speeds without risking burnt exhaust valves after a while. The D8ES-L/DR8ES-L or X24ES-U/XR24ES-U are all suitable for this type of riding.
