I guess I should have provided more background... It's a stock CB750 K5 with a 4 into 1 pipe. I have not rode the bike for a month (rain, cold, life, etc.). Bike is new to me as of Sept/Oct. Conditions just prior to last shutdown were likely fully warmed up engine having romped around town, idle a few seconds up the driveway then shut down. Plugs have a few hundred miles on them. Not exactly scientific, just thought the very clean looking 2-3 plugs were odd, as if not firing consistently and getting washed down with fuel or possibly just lean.
It could be vacuum leaks, though boots are soft and new(er) from PO. One of the clamps was too large and I swapped for a smaller one that tightens properly. I will spray around the boots just to be sure though.
I have had 3 sets of idle jets stamped 40 in these carbs. The first was way lean at idle, the second was way rich... At that point I started eyeballing them closer and noticed the first set that came in the carbs while appearing brand new appeared to have a very small hole. The second set came off a rack on a pretty hot engine that ran well with 140 jets. These looked very large, and as it turns out were drilled. No good for a stock K5 engine. Compared a couple other sets of slow jets from other racks and found the rest to appear consistent in size so cleaned up a set and threw them in, now it starts easier cold and idles better, and mixture screws have some effect where before they had little/no effect.
Regarding the Dyna. I seem to recall measuring the old spark plug caps at around 9k-10k ohms. I will measure them today or tomorrow for some postmortem. I do have new caps, I think one set is resistor and another is non resistor. The plugs I am running are NGK DR8EIX, iridium with 5k resistor. I have runs these plugs with a Dyna S, 3 ohm coils, and solid wires with no resistor caps on 4 CB750s now and never had a problem with insufficient charging or dead battery. Noteworthy items may be that I rarely use the electric start (though it always works when I do) and I don't have any other additional electric loads. Also, this bike is a K5, the other bikes I've used this setup on have been K1 and K2, not sure if the later bikes had any charging system differences which could result in problems?
I'm content to run it as is since it has worked for me... If there are charging issues I can add the 2 ohm resistor pack you offer. Is the 5k ohms in the plugs themselves sufficient or should I be adding resistor caps as well? If so what total resistance would work best? Also I will check voltage at idle and various RPM to assess charging.
OK, that helps a little bit!
The post-1973 750 bikes produce a little bit more power because their alternator rotors were more precisely made. The advertised wattage has always been 210w, but most make about 160-180 when hot.
Might it be: the dark sparkplug came from the cylinder with the incorrect clamp?
The resistance situation: the OEM plug caps on the K4/5 bikes were 7500 ohms, and are considered burned out if either the resistance on the 2 of one coil are different by more than 500 ohms, or if they are above 8500 ohms. They get higher in resistance as they age, because the little carbon pellet burns off a bit at a time, burning the contacts inside as it does. This is entirely normal. Today we can get 0 ohm, 5000 ohm, and 10,000 ohm caps (although the latter are hard to find, for some reason). The "05" in the numbers I cited above are the "5K" value (5000), while the other parts numbers have either "00" or "10" in those 2 digits, for 0 ohms or 10k ohms, respectively.
Here's the story behind the resistance: as the coil discharges its spark, the amount of current that passes during the spark determines how fast the discharge occurs. Inserting resistance in the path makes the spark last longer. For example, using the numbers I measured during the development of the Transistor Ignition (here in these forums), the discharge time of the OEM coils with 0 ohms was about 1.05mS at 5000 RPM, 1.45mS with 5000 ohm plug caps, and 1.70mS with 10,000 ohm plug caps. It looks from this data that Honda "split the difference" between the 5k and 10k values, as the shorter increase between 5k and 10k vs. the 0 ohms and 5k values shows the coils were maxing out somewhere between those 2 upper values. (This has to do with how much inductance was wound into the particular TOYO coils that they used.) Today, we can approximate the 7500 ohm 'nirvana' value by using the 5000 ohm caps and the typical resistor sparkplugs, most of which are in the 2000-ohm range (a few are higher, usually intended for EFI closed-loop automotive systems that vary spark and fuel via software, in real time).
When the coil inductance gets lower (i.e., 3-ohm coils) the resistance must get higher to prolong the spark discharge. In these engines, unless you build them past 10.5:1 compression, they only need 4500 volts to spark sufficiently to run well, cold or hot. The OEM coils make about 7500 volts at 12 volts power (most get only 11.0 volts, making about 6000 volt sparks in the process), so their excess capability is almost 80%. The Dyna 3-ohm coils make 30kV at 5000 RPM into a 0.050" gap, but the discharge is a pitiful 0.9mS long (i.e., less than 1mS). In addition, the Dyna S triggers short-circuit the last 40% of this spark above 5500 RPM, on every firing, because they switch back on too soon, interrupting the spark and heating the coils.
This, specifically, was why they often burned themselves out when run in high-speed touring in the "old days" when we ran back-to-back-to-back 1000 mile days for sometimes 2 weeks at a time. I burned out 2 coils (1 in each pair) and 2 Dyna S units (one of those running OEM coils) in 3 years, and quit using them afterwards. I hate pushing my bike...oh- and also a Dyna III unit, which was Dyna's first answer to the 'too-short dwell' question, circa 1974. This little box went between the Dyna S and the coils, stretching the OFF time a little bit in the process, to cool off the coils. But, the boxes frequently died on the hiways, and I haven't even seen a working one in 20 years(!). At least at the time we could unplug it and run from the Dyna S triggers to get home!
To cool of the "hot" coils today, and improve the engine performance at the same time, here's the tips:
1. Use resistor caps AND resistor plugs. This also helps to stretch the discharge time, attaining nearly 1.15mS if the caps are 5000 ohms and the plugs are 2500 ohms. If you can find the 10k plug caps, go for it! You might reach 1.2mS, which you will notice at speeds above 60 MPH.
2. Make sure the gaps on the Dyna triggers (between the pickups and the magnet) are exactly equal, and in the midrange where they work. This takes some time to set up: with the engine warmed up, first move the pickups (one at a time is OK) away from the magnet until it won't fire, then repeat TOWARD the magnet. Measure each position with a feeler gage, add those together, divide by 2, and set them at that distance. THEN time them, using a strobe timing light. This will provide the longest-possible OFF dwell time at the full RPM range, with it running longer at low speeds and getting shorter at high RPM, particularly above 5500 RPM.
3. When you're ready, get a Resistor Pack. This will simultaneously cool off the coils, the triggers, extend the spark another 0.5mS, and lighten the load on the alternator, Run-Off switch, and keyswitch wiring. This will let the system voltage rise, which in turn will improve the spark, particularly in heavy city traffic after 2 hours of riding time (typically).
Happy New Year! Stay warm.
-HM
