Those sparkplugs: here's the skinny on them -
The post-1975 SOHC4 bikes came OEM with 10,000 ohm sparkplug caps. (The earlier ones were 7500 ohms.) This was done for 2 reasons: one was [obstensibly] to make Honda look more like the 'nice guys' by reducing interference to nearby car radios (AM radio) while sitting in traffic in the 1970s. The second, and far more plausible, reason was: they needed a longer spark duration in the hemi-shaped combustion chambers of the post-1975 head designs on all of these bikes. Adding resistance to the plug caps was the easiest way to get it back then, so they also changed the coils in 1975 on ALL of these bikes to be 4.3 ohm primaries (instead of the earlier 4.6 ohm typical value) to decrease the turns ratio and increase the spark voltage a little (almost 1000 volts). Between these 2 changes, the spark duration came out the same as in the earlier bikes, but with about 800 volts more spark voltage at the plugs. This was needed because the cams were moved back about 5 degrees (intake open measurement) to meet EPA emissions regs at the time, which made the power appear later in the RPM range (5500 instead of 4700-ish) and caused a new cold-blooded nature to appear.
Today, we can't find the 10,000 ohm plug caps for nuthin'. The most we can usually do is get the 5000 ohm types. But...if you also add the 2000 ohm sparkplugs (DR8ES-L or XR24ES-U) then you can approach the earlier 7500 ohm spec, in which case you can use the earlier coils and that combo to get the longer-duration spark back. That's a pretty good thing!
The original 1969-1970 sparkplug for these bikes was the D8ES. This was a plug that could handle heat up to 10,000 RPM ranges while staying clean at D7E-type (cold environment) temperatures, and stay clean. The EPA forced ND/NGK to stop making these sparkplugs in 1998, stating that "we (the EPA) do not believe that these sparkplugs can meet that wide of a heat range...", and forced them to increase the ceramic-to-metal joint surface so the plugs would over-cool when used at lower engine speeds. This makes them foul...
When Honda first faced this too-cold challenge in 1970 due to so many inexperienced riders riding sedately (think: bankers and lawyers who never rode before, but bought a "cool" 750 to putt to work), they needed a hotter-but-not-too-hot (like D7ES) solution. They also leaned out the carbs a bit and asked NGK to come up with a heat range between the too-hot D7ES and the D8ES. They made it the D8ES-L, with a longer tip to stick further into the chamber, and cool itself slightly less. This helped!
... then the US government so 'wisely' lowered the US speed limits to 55 MPH overnight in 1973... the CB750, which was the #1 bike on the roads at that time, suddenly found itself fouling the D8ES sparkplugs in 1000 hiway miles for lack of RPM to clean them off and heat them up. Even the D8ES-L was having troubles at this speed. To make a splash, NGK's parent company ND then created the X24ES-U, which was similar but with an even thinner tip, and less ceramic-to-metal contact in the body so it retained more heat. To set it apart from everyone else, they grooved the ground arm into a tiny "U" shape, hence the name. This latter one runs like an imaginary D7.6ES (or maybe 7.
heatrange, and is perfect for all but flat-out racing apps. I have used them almost exclusively on my 750(s) since then. To control the slightly-hot tendency at 85+ MPH touring speeds (in places like Wyoming and Nevada) I use premium gas, while running midgrade the rest of the [high-country touring] time and low grade in town. They stay clean, start great, and some of the ones I have date to 1974, having been cleaned as many as 5 times!
