Topic: CB400F coil upgrade - any recommendations?

[CB400Finhawaii]

Aloha,

I have a 1975 CB400F that hasn't run in about 20 years.  I finally got it started after about 18 months of on and off work on the bike.  The bike ran pretty rough and did not idle or rev well.  I had rebuilt the cards.  It has a new battery.

I checked the valve clearance and they were fine.
I checked the timing using a static light, but could not get the timing close. 
I had spark on all cylinders when I first got the bike running, but do not have spark on all cylinders now.  Seems like which cylinders have spark is not consistent when I check.

To (hopefully) solve the timing/rough running  issue, I installed a Dyna S electronic ignition.  I am sure it is installed correctly and the timing is right on.
So I think I should upgrade my coils.  Also, the insulation in the wire from the coli to the spark plug cap on 4 keeps crumbling.  I have cut the wire back a couple of times hoping to get a good connection, but the insulation keeps crumbling with each new cut.  The spark plug cap just won't stay on the wire, and the wire is pretty short now. I believe it is all original parts. So I think I should replace all the wires and spark plug caps, and coils while I am at it.

I am hoping that new coils are the answer.  I will check all the connections under the tank and to the coils as well.

- Would you go with stock coils or upgrade the coils?   If upgrade, what brand and who would you buy from.  I get most of my parts from 4into1.com, but their Magna 5 ohm coils do not fit the stock bracket on the CB400F.  Anyone have experience using their Manga coil and fabricating your own bracket to make it fit?   The kit with wires, cap and coils is $84.  Anyone use Dyna coils?  They are pretty pricey at around $200 a pair.  Anyone use any other brands?

Also, the Dyna S says the coils should be at least 3 ohm.  Should  I go with 3 or 5 ohm coils?  Any other suggestions and recommendations?  Anything else that i should be checking?

Thanks

[bryanj]

If you use 3 ohm you will never keep the battery charged, 5 ohm was standard

[Deltarider]

If you use 3 ohm you will never keep the battery charged, 5 ohm was standard

I have proved by measuring*, that this ^ is a myth. People have concluded this by simply deducting Ohms law. Dynamically seen (= running), things are a bit different. I have had 3Ω coils for decades and still run them. The difference in current drawn dynamically is negligible. Do I then recommend 3Ω coils? No! They bring nothing. When you have your engine idling a lot, you may expect a fraction more pitting of the breakerpoints, but TEC points can handle that no prob. All in all you can stick to stock.
If you're interested, read this: http://forums.sohc4.net/index.php/topic,188940.msg2195751.html#msg2195751
Especially in the field of invisible phenomena, like eiectricity and magnetism, people tend to fantasize and attribute all kinds of ideal properties to EI ignitions like Dyna. In practice a stock ignition is more than adequate.
* http://forums.sohc4.net/index.php/topic,175144.msg2038035.html#msg2038035

[PeWe]

My CB750 K6 drained battery very quick with Dyna 3 ohm coils and Dyna-S ignition.

I could return coils to the shop and got Dyna 5 ohm coils for a low fee.
I had used them during an 1 hour ride.

Much better with 5 ohm coils.
I did not try to discuss the matter with the bike that should know better!

[willbird]

I bought the 5 ohm coils from ebay that take an automotive type spark plug wire that you can swap out. Then bought a universal spark plug wire kit from Tractor supply. Some do not like the coils where wires plug in but they have been fine for me.

The  TSC, (actually it was a smaller non chain farm store) are copper core.

[Scootch]

I have Accel coils on my 750 K0. They are 3 ohm. I made my own plug wires. They are stranded copper center conductor with crimped end connectors. Did not like the graphite core wires with push/screw in caps. Unreliable connections with the screw in caps.

[Deltarider]

Graphite core wires are a no no as far as our bikes.

[Stev-o]

There have been many reports that guys think they have bad coils, purchase new aftermarket coils but still have an issue.

Most likely, the stock coils you have are fine and you that have another issue.

[Bodi]

Maybe there are 400F/350-4 bikes that can charge with a Dyna S and 3 Ohm coils. It defies physics, but...
The Dyna S powers the coils all the time, except for a very short time when the spark is triggered. Points are different, the coils are powered for about 60% of a revolution so right there power draw is higher with the Dyna. Then the 3 Ohm coils draw quite a bit more than 5 Ohm, regardless of "dynamics" the resistance and impedance are lower and they will take more power.
350-4 and 400F alternators are not blessed with a lot of excess power output over what a stock system draws. Using 3 Ohm coils the battery might still charge. Plus a Dyna S, doubtful. Add a higher wattage headlight and it's hopeless IMO.
The battery will still recharge a bit but not fully. The system voltage will never get high enough. What happens is that the alternator can only put out it's maximum amps. All the loads are essentially resistive so amp load is proportional to voltage (Ohm's law is strictly enforced). The system voltage will settle at whatever the alternator's power output equals the load power draw.
With a stock load, after starting and then a bit of riding the battery will fully charge, and the regulator will limit the voltage at 14V or so: full charge voltage. Overloaded with mods the voltage may only get to 11V or so with the regulator never reducing alternator output and the battery only getting partially charged.
Plus: the alternator is an induced field type, with the magnetic field produced by a field coil. Field strength is proportional to coil voltage, and alternator output is proportional to field strength. As the system voltage drops, the alternator output drops. Too much load and the system voltage keeps dropping as the battery discharges reducing system voltage reducing alternator output and dropping voltage more - at some eventual low voltage the ignition can't spark and you're stopped.

[willbird]

Maybe there are 400F/350-4 bikes that can charge with a Dyna S and 3 Ohm coils. It defies physics, but...
The Dyna S powers the coils all the time, except for a very short time when the spark is triggered. Points are different, the coils are powered for about 60% of a revolution so right there power draw is higher with the Dyna. Then the 3 Ohm coils draw quite a bit more than 5 Ohm, regardless of "dynamics" the resistance and impedance are lower and they will take more power.
350-4 and 400F alternators are not blessed with a lot of excess power output over what a stock system draws. Using 3 Ohm coils the battery might still charge. Plus a Dyna S, doubtful. Add a higher wattage headlight and it's hopeless IMO.
The battery will still recharge a bit but not fully. The system voltage will never get high enough. What happens is that the alternator can only put out it's maximum amps. All the loads are essentially resistive so amp load is proportional to voltage (Ohm's law is strictly enforced). The system voltage will settle at whatever the alternator's power output equals the load power draw.
With a stock load, after starting and then a bit of riding the battery will fully charge, and the regulator will limit the voltage at 14V or so: full charge voltage. Overloaded with mods the voltage may only get to 11V or so with the regulator never reducing alternator output and the battery only getting partially charged.
Plus: the alternator is an induced field type, with the magnetic field produced by a field coil. Field strength is proportional to coil voltage, and alternator output is proportional to field strength. As the system voltage drops, the alternator output drops. Too much load and the system voltage keeps dropping as the battery discharges reducing system voltage reducing alternator output and dropping voltage more - at some eventual low voltage the ignition can't spark and you're stopped.

I got interested in coils, points, and alternatives and started to look at the actual situation that goes on. IMHO there is no way that the Dyna S can be charging the coils "all the time". The current draw would be massive and the coils would not live even a few hours.

With a setup like a Microsquirt coil dwell is often 2.5 to 3.5 milliseconds and when I ran the numbers on a points ignition I expected to see a LOT higher dwell time but the answer was nope. Quick googling rather than doing the math shows me 23 milliseconds dwell at 800 rpm may be typical for automotive stuff. That number gets divided by 10 at 8000 rpm IMHO so dwell then is 2.3 milliseconds on the same system. Again for points. If the coil was "on all the time" each 5 ohm coil would draw 2.4 amps.

I'm sure HondaMan has some real world ms dwell numbers.

[Deltarider]

[...]
The Dyna S powers the coils all the time, except for a very short time when the spark is triggered. Points are different, the coils are powered for about 60% of a revolution so right there power draw is higher with the Dyna.

True, be it that with points correctly set, Duty Cycle will be between 51 and 54%.

Then the 3 Ohm coils draw quite a bit more than 5 Ohm, regardless of "dynamics" the resistance and impedance are lower and they will take more power.

They take just a fraction more. Anyone can do the test I did: measure the amps between the kill switch and a set of 3Ω coils and compare the outcome to a dito measurement with standard 4,5Ω coils. With the engine running ofcourse, both at idle as well as revved.

350-4 and 400F alternators are not blessed with a lot of excess power output over what a stock system draws. Using 3 Ohm coils the battery might still charge.

Yes, it will. But... there's no need for 3Ω coils. (Re)read reply #2. It has all the info.

  Plus a Dyna S, doubtful.

Yes. But who needs an extended dwell period when Honda has dimensioned their coils thus that saturation will be enough to guarantee good ignition at all rpms.

Add a higher wattage headlight and it's hopeless IMO.

Could be. I don't know if anyone has tried this on a CB400F. The owner of the CB350F-MM in the pic had no problem. With a stock ignition or an EI that has the same dwell (!), I doubt replacing the standard headlight bulb by a H4, will be much of a problem.

[...] All the loads are essentially resistive so amp load is proportional to voltage (Ohm's law is strictly enforced). [...]

I'm afraid I can't follow you here. Could you explain a bit more?

[Mark1976]

If you use 3 ohm you will never keep the battery charged, 5 ohm was standard

I have proved by measuring*, that this ^ is a myth. People have concluded this by simply deducting Ohms law. Dynamically seen (= running), things are a bit different. I have had 3Ω coils for decades and still run them. The difference in current drawn dynamically is negligible. Do I then recommend 3Ω coils? No! They bring nothing. When you have your engine idling a lot, you may expect a fraction more pitting of the breakerpoints, but TEC points can handle that no prob. All in all you can stick to stock.
If you're interested, read this: http://forums.sohc4.net/index.php/topic,188940.msg2195751.html#msg2195751
Especially in the field of invisible phenomena, like electricity and magnetism, people tend to fantasize and attribute all kinds of ideal properties to EI ignitions like Dyna. In practice a stock ignition is more than adequate.
* http://forums.sohc4.net/index.php/topic,175144.msg2038035.html#msg2038035

   First hand experience here, the 400f cannot support a pair of 3 ohm coils and a dyna ignition. I've done it, it doesn't work, you end up with a dead battery and overheated power leads that feed the coils. Next, I dumped the dyna ignition kept the coils, same thing, under charged battery. There's just not enough charging capacity to support the 3 ohm coils.  Sooo, I dumped everything dyna and replaced with the stock points and coils, ran fine, charged fine, SOB!!!!
   In the end I kept the points, used hondaman's ignition upgrade (ditched the condenser's), went with dyna 5 ohm coils and wires and use 5 ohm plug caps. My bike starts at a touch of the starter button every time. My bike is hardly stock, but its not crazy built either, currently wearing a kerker, web cam and springs, completely reconditioned head, hi volume oilpump, it sees its share of rpms every time I ride it. Same battery now last 6 yrs.
   Sold the dyna ignition with my 550f a couple of yrs ago, it worked well on the 550f, but it ran just as well with the points system. It maybe a set it and forget it affair but it didn't really improve the performance any measurable amount.

[carnivorous chicken]

There have been many reports that guys think they have bad coils, purchase new aftermarket coils but still have an issue.

Most likely, the stock coils you have are fine and you that have another issue.

This. In my experience it's very rare that the stock coils go bad and are the issue.

[The Lone Builder]

HT wired on the coils can be replaced with a bit of effort.

There are two options, which are essentially the same:

https://www.dropbox.com/s/x0a4fcpmrwm3l19/Ignition%20coil%20fix.doc?dl=0 - This is the original, and

https://www.sohc.co.uk/index.php?topic=14939.0 a slight modification.

I have done this on CB400 coils and it works a treat.

[Tim2005]

I use 3 ohm coils on my 400Fs, with points or Boyer ignitions, no problems. I've done this since the 1990s, as the only coils you could get locally with replaceable leads were 3 ohm (& I had no idea about ohms, I just wanted HT leads that didn't rot away). It was only since the internet appeared that I discovered that they perhaps shouldn't be ok.

[HondaMan]

(Ohm's law is strictly enforced).

Yeah, and his lawyers are REAL good.

[Deltarider]

Reread reply #11 and #14. Noticed the difference? Well, Mark1976 is in the US and Tim2005 in the UK. Telling, isn't it? It's exactly what I have stressed in all those years of discussions here on Honda's charging 'shortcomings'. The Dutch owner of that CB350F-MM in the pic had no charging problems.