Topic: Measuring ohms on sparkplug caps.

[HondaMan]

The SOHC4 bikes all came with "resistor caps" for the sparkplug connections.
On the bikes prior to 1976, the ohms value of the resistor inside was 7500 (7.5k) ohms from the factory. Over time, they slowly burn out and increase in value: at 9500 (9.5k) ohms, they are considered to be "burned out" and can cause several problems, including hard starting and fouled sparkplugs. This can also make the spark jump to the engine or frame (or your leg, in the rain), making for some pretty night effects. 

From 1976 until the end of the SOHC4 series, the ohms value was 10,000 (10k) from the factory, and the coils came with thicker spark wires because of this higher resistance (to prevent pretty night effects!). This later design was intended to reduce AM radio interference for cars next to your bike, and for riders who were sporting fairings with built-in radios for touring. It also made the plug cap life shorter, as the "burnout limit" was (is) only 11,000 (11k) ohms. These coils can be retrofitted to the earlier plug cap resistances with improved spark being the result, as the internal parts of the coils did not change.

Today, we can get plug caps from NGK in 5000 (5k) ohm values, which will slowly burn out over time to the 9500 (9.5k) limit over a much longer period. The newer plastic and rubber boots are superior quality: a bargain for their $5 price tag. They come in the straight, 90 degree, and 135 degree angled shapes for all of the SOHC4 bikes. Use these as replacements for superior performance on your bike.

NOTE: NGK makes caps in 0 ohms, 5000 (5k) ohms, and 10,000 (10k) ohms types. Pick the right one!

An alternate approach for the CB750, CB350F/400F and the CB650 bikes, and touring with the CB500/550, is to use the 0 ohm caps from NGK and the resistor sparkplug DR8ES-L from NGK. This heatrange is ideal for today's gasolines that are laced with ethanol: it is halfway between the colder D8EA that fouls easily and the D7EA that fits the CB500/550 engines. Don't use both 10k resistor caps and this resistor-type sparkplug, though, or you will have too much resistance in the spark circuit. There must be at least 5000 ohms in this circuit, whether you use Honda or Dyna or Accel coils, to make things work properly. The resistor-type sparkplugs are about 2000 ohms.

Below are some pictures of a cap I broke open so you can see a resistor from inside, and a shot of how to measure the caps with a digital ohmmeter. The cap shown is a burned-out CB750K4 cap (1974 build), registering 10.5K ohms.

Set your ohmmeter's range switch to register in the appropriate ohms range first. Then touch one meter probe to the little screw tip inside the wire end of the cap, and the other probe to the spring socket where the sparkplug snaps in to the cap. Polarity makes no difference, here.

[BlindJoe]

maybe this should go in the electric FAQ too

[totalshutdown]

Not sure if this thread is dead or not but I'll give it a shot. What about the wires themselves? Should they be replaced as well? What about testing the resistance of them and would the wires from NGK work? The local shop sells bulk wire for 1.75 a foot.

[RAFster122s]

You need the copper core wire, not the carbon core like in modern auto plugwires...

David

[HondaMan]

The wires in the stock coils are not replace-able. 
Once in a while they will burn in two at the upper end, where the multiple strands are spot-welded to the single wire coming out of the coil. It's just the years of wear (spark erosion and twisting of the wires while changing plugs) there that cause this: the typical symptom then is severe cold-bloodedness of the affected cylinder, with gas fouling of the OTHER plug in the pair because it gets weak spark, while this one gets little spark at all and usually appears wet and clean. You can test your wires for this with a tug on the wire: if it comes out, it's toast.

I've been recently experimenting with the Dyna 3-ohm Green coils and have found that using the 10k ohm caps (see post above) can stretch the spark duration almost 10% longer with the plugs gapped at 0.045". This is a good thing, especially in the swirl-charge engines like the 750 K0-K6. 

[Seeparkfly]

The wires in the stock coils are not replace-able. 
Once in a while they will burn in two at the upper end, where the multiple strands are spot-welded to the single wire coming out of the coil. It's just the years of wear (spark erosion and twisting of the wires while changing plugs) there that cause this: the typical symptom then is severe cold-bloodedness of the affected cylinder, with gas fouling of the OTHER plug in the pair because it gets weak spark, while this one gets little spark at all and usually appears wet and clean. You can test your wires for this with a tug on the wire: if it comes out, it's toast.

I've been recently experimenting with the Dyna 3-ohm Green coils and have found that using the 10k ohm caps (see post above) can stretch the spark duration almost 10% longer with the plugs gapped at 0.045". This is a good thing, especially in the swirl-charge engines like the 750 K0-K6. 

Will this Dyna 3-ohm Green coil setup and gap work with my 76 CB750F? Thinking about changing the stock coils later down the line.

I have stock coils will the 5,000 ohm caps do? It has 135 degree caps on the ends and two straight ones in the middle. I'm guessing that's normal since it's that way on the K3

Also, a couple of months ago I bought your ignitions for the K3 but didn't install because I knew I'd get the F1, back then you recommended the the Denso U-groove spark plugs. Are these good for the F1?

Cheers

[jchabino]

Thanks Hondaman.  This really helped me out.

[HondaMan]

The wires in the stock coils are not replace-able. 
Once in a while they will burn in two at the upper end, where the multiple strands are spot-welded to the single wire coming out of the coil. It's just the years of wear (spark erosion and twisting of the wires while changing plugs) there that cause this: the typical symptom then is severe cold-bloodedness of the affected cylinder, with gas fouling of the OTHER plug in the pair because it gets weak spark, while this one gets little spark at all and usually appears wet and clean. You can test your wires for this with a tug on the wire: if it comes out, it's toast.

I've been recently experimenting with the Dyna 3-ohm Green coils and have found that using the 10k ohm caps (see post above) can stretch the spark duration almost 10% longer with the plugs gapped at 0.045". This is a good thing, especially in the swirl-charge engines like the 750 K0-K6. 

Will this Dyna 3-ohm Green coil setup and gap work with my 76 CB750F? Thinking about changing the stock coils later down the line.

I have stock coils will the 5,000 ohm caps do? It has 135 degree caps on the ends and two straight ones in the middle. I'm guessing that's normal since it's that way on the K3

Also, a couple of months ago I bought your ignitions for the K3 but didn't install because I knew I'd get the F1, back then you recommended the the Denso U-groove spark plugs. Are these good for the F1?

Cheers

The wires in the stock coils are not replace-able. 
Once in a while they will burn in two at the upper end, where the multiple strands are spot-welded to the single wire coming out of the coil. It's just the years of wear (spark erosion and twisting of the wires while changing plugs) there that cause this: the typical symptom then is severe cold-bloodedness of the affected cylinder, with gas fouling of the OTHER plug in the pair because it gets weak spark, while this one gets little spark at all and usually appears wet and clean. You can test your wires for this with a tug on the wire: if it comes out, it's toast.

I've been recently experimenting with the Dyna 3-ohm Green coils and have found that using the 10k ohm caps (see post above) can stretch the spark duration almost 10% longer with the plugs gapped at 0.045". This is a good thing, especially in the swirl-charge engines like the 750 K0-K6. 

Will this Dyna 3-ohm Green coil setup and gap work with my 76 CB750F? Thinking about changing the stock coils later down the line.

I have stock coils will the 5,000 ohm caps do? It has 135 degree caps on the ends and two straight ones in the middle. I'm guessing that's normal since it's that way on the K3

Also, a couple of months ago I bought your ignitions for the K3 but didn't install because I knew I'd get the F1, back then you recommended the the Denso U-groove spark plugs. Are these good for the F1?

Cheers

I dislike the 3-ohm coils in these bikes, as they use more electrical power than most of the bikes can generate. The rare 750 that can support them is usually found in freeway service or touring, but even then the battery can run low if the headlight is halogen bigger than 60w hibeam and the bike is used in city riding for more than 45 minutes at a time.

A better bet is to use either the Honda coils or the Dyna 5-ohm coils, or, if you can find some, the Sakura coils sold by PartsNmore (which are very good). The latter requires a little wire-color-trading and connector-swapping, but work perfectly.

[RAFster122s]

Mark,
 Would those be the partsmore #17-6823 coil for around $40?
They are listed for cb750 use. Would they be usable for the 550?
Thanks!
David

[HondaMan]

Mark,
 Would those be the partsmore #17-6823 coil for around 40?
thry are listed for cb750 use. Would they be usable for the 550?
Thanks!
David

Yep, they work great! I've used them on the 350F/400F/550 and 750 all.

[camelman]

Mark, I'm rebuilding a CB200t for a guy as trade for a different bike and was struggling with cap resistance requirements. Lo and behold, the best answer on the web comes from you! I really need to buy your CB750 book. 

Thanks for sharing your knowledge.

Cheers,
Camelman

[HondaMan]

Always glad to help!
It's about riding...

[bytio]

Mark
Excellent information thanks. Have you had any experience of the Boyer Bransden micro digital system  and coils? ( these were suggested as a replacement for original coils)

Thanks
David

[HondaMan]

Mark
Excellent information thanks. Have you had any experience of the Boyer Bransden micro digital system  and coils? ( these were suggested as a replacement for original coils)

Thanks
David

The only Boyer parts I used were decades ago. They were then an optical (photoeye) trigger system. Mine died, so I didn't ever use them again. Points and my Transistor Ignition provides a backup: the Boyer required a pickup truck to get the bike home. For me, that's simply not acceptable.

[ultrav]

Since this topic has stayed somewhat alive. I am in the process of trying to tune up the old girl ('81 cb650C) with some new ignition components. Hondaman, can you possibly suggest what coil resistance, wire type (copper i assume), caps resistance, & plug type would probably benefit her the best? I've tried to understand all of the information here but it's a mix of points ignition and CDI and I just want to make sure.

Thanks in advance!

Brian

[HondaMan]

Since this topic has stayed somewhat alive. I am in the process of trying to tune up the old girl ('81 cb650C) with some new ignition components. Hondaman, can you possibly suggest what coil resistance, wire type (copper i assume), caps resistance, & plug type would probably benefit her the best? I've tried to understand all of the information here but it's a mix of points ignition and CDI and I just want to make sure.

Thanks in advance!

Brian

The later 650 bikes used the CDI with coils of about 2.8 ohms primary resistance. This generates about 28kV spark voltage, but in a VERY, VERY short spark. This is one of the things that gives the 650 that "thin" feeling in the power: the fuel only burns about 1/3 the way down the bore because the spark is too short to ignite a wide flamefront and total burn.

To stretch the spark, Honda used 10k ohm caps on that bike OEM. That will help somewhat. If you can find those caps (used on several modern superbikes, BTW) that's the first recommendation I would give. In addition to that, I would add resistor-type plugs in the same heatrange you use now. This will also stretch it a bit more. If your coils are the ones that have removable wires as well, consider using a silicone-core, resistor-type spark wire: this will only add 200-500 ohms because of the short length, and can make the wires susceptible to losing some spark energy when soaking wet, but after drying out they will be OK again.

Many of the CDI units I have seen on these bikes have really dirty connectors in the plug to the CDI box. This really drops the power to it, as it draws its power in surges. Clean those well, add a drop of oil and then plug them together tightly. If the plastic is dying, maybe look at Del's offerings at www.vintageconnections.com and see if he has the right plug to match up a new one?

Although, if I were taking a 650 racing, I'd see about simply removing the plug and just soldering the wires to the terminals. This will make much less heating and voltage loss - but on the road, it's terribly inconvenient!

[ultrav]

I've got a set of replacement coils on there right now as far as I know since they have the replaceable wires. They are reading 2.4 ohms right now. The caps that the PO put on the wires were only 5k ohm with regular D8EA plugs. I've experienced what appeared to be fuel fouling a bit (plugs were wettish) and really rich smelling exhaust and some bogging at WOT in higher RPMS.

I have found coils that are rated for 4 ohms from dime city, should they be alright or should I go with the 17-6803 coils from PartsnMore?

I'm having a heck of a time finding 10k caps so I might be stuck with the XD05F caps that have been on it. Match them up with the DR8ES-L plugs you referenced earlier and that should act like a 10k at that point correct?

Thanks a million for the input.

[HondaMan]

I've got a set of replacement coils on there right now as far as I know since they have the replaceable wires. They are reading 2.4 ohms right now. The caps that the PO put on the wires were only 5k ohm with regular D8EA plugs. I've experienced what appeared to be fuel fouling a bit (plugs were wettish) and really rich smelling exhaust and some bogging at WOT in higher RPMS.

I have found coils that are rated for 4 ohms from dime city, should they be alright or should I go with the 17-6803 coils from PartsnMore?

I'm having a heck of a time finding 10k caps so I might be stuck with the XD05F caps that have been on it. Match them up with the DR8ES-L plugs you referenced earlier and that should act like a 10k at that point correct?

Thanks a million for the input.

The 4 ohm coils will be to slow to charge with the CDI system. That system needs those low-ohm coils. look a little further at PartsNmore for the 1980-82 CB750 coils, or maybe at 4into1.com (they have sometimes sent the 2.8 ohm coils incorrectly in place of the 4.5 ohm types, so I know they have them!).

The resistor-type sparkplugs are about 2000 (2K) ohms, so they will help a little bit with the 5k caps. In that case, if you have replaceable wires, just get an old car wire somewhere and cut it to length(s) to fit these shorties. This will add a little bit more, which will help.

Tip, if you don't already know: the 10k caps are numbers like XD10F, SD10F, etc.

[ultrav]

Gotcha. If I can source some XD10F caps, you think I should still use the resistor plugs too correct?

[HondaMan]

Gotcha. If I can source some XD10F caps, you think I should still use the resistor plugs too correct?

On the 650, yes, I would IIWM. Those ignitions make plenty of 'excess spark energy', so the only limitation to resistance will be when it starts jumping to the engine in the wet, instead of through the sparkplug. The discharge duration will get longer and longer: if you can reach the old 'nirvana' of 1.5mS, it will again match the 750/500 systems. The result will be noticeably increased midrange with no loss of top-end power, for all your trouble.

[JerryWayneCattle]

The wires in the stock coils are not replace-able. 
Once in a while they will burn in two at the upper end, where the multiple strands are spot-welded to the single wire coming out of the coil. It's just the years of wear (spark erosion and twisting of the wires while changing plugs) there that cause this: the typical symptom then is severe cold-bloodedness of the affected cylinder, with gas fouling of the OTHER plug in the pair because it gets weak spark, while this one gets little spark at all and usually appears wet and clean. You can test your wires for this with a tug on the wire: if it comes out, it's toast.

I've been recently experimenting with the Dyna 3-ohm Green coils and have found that using the 10k ohm caps (see post above) can stretch the spark duration almost 10% longer with the plugs gapped at 0.045". This is a good thing, especially in the swirl-charge engines like the 750 K0-K6. 

Hello. Dead post. I know, I know. But, wondering what setup ended up working best with the 3ohm coils? So a question for Hondaman I suppose. I know you dislike the 3ohm coils due to power draw, but other than that was the 10k resistor cap and 0.045 gap the optimal combination that you found?

I run the 3ohm coils with my 849 cycle x kit (I bought this motorcycle from member "banzaibob") and they seem to work well. I am mainly highway and charging has not been an issue. I've been using 5k resistor caps, but just noticed one was reading at almost 20k! So instead of just replacing the one 5k I thought about possibly doing them all with 10k after remembering you writing about the caps prolonging spark duration.

I hope you see this and thank you for all of the information you have posted over the years. I have learned so much about these beautiful machines just by simply reading these forums.

EDIT: Or maybe DPR9EA-9 plugs (I use d9ea currently) with 5k caps? The 10k caps are proving to be very hard to find. At least at a price under $30

Jared

[HondaMan]

The 3-ohm coils aside from their too-hungry use of the bike's meager power supply, make a VERY short duration spark. This hurts performance across the whole RPM range in theses SOHC4 engines. Even worse, if you use a PAMCO or Dyna S ignition, the spark duration gets even shorter at higher RPM.

These engines need about 4500 volts of spark. Anything more than that is just wasted. If you raised the compression with some 11:1 Henry Abe pistons, then you need about 6000 volts of spark.

So, the 3-ohm coils serve no useful purpose on these bikes, and in the process they drain the electrical system, overcook the wiring and bullet connectors in the Black/White wire (and Run-Off switch) circuit to the point of even melting the Run-Off switch contacts, and eventually self-destruct. Last week the last (4th) Dyna 3-ohm coil I use for my Transistor Ignition test bench split open (loudly!) during a test session: I don't have any more of them and do not plan on buying one, either. I'll find something better to test my ignitions with.

[JerryWayneCattle]

Right on. Thank you for the reply.  Looks like I'm just gonna go back to 5 ohm coils with stock plug gap. I hated how things were seeming over complicated anyhow

[HondaMan]

Right on. Thank you for the reply.  Looks like I'm just gonna go back to 5 ohm coils with stock plug gap. I hated how things were seeming over complicated anyhow

You, and your Honda, will be much happier in the end.

[nickfarrowdesign]

Just discovered this great thread and it's made things a bit clearer on my 750 k3 build using standard points.

So what I can deduce is that with my new 5k Ohm coils I should be using 5k Ohm spark caps and resistor type plugs?

Would hate to buy the wrong parts so if anyone can confirm, I'd be indebted! NGK numbers for caps and plugs would be amazing too but maybe pushing my luck...

Cheers guys, stay upright!



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