Topic: Spark arcing from spark plug cap

[scottly]

Are you using anti-seize on the plugs?

[papi_sosaa]

Just throwing this out there but could failing CDI units lead to this as well?

[papi_sosaa]

Are you using anti-seize on the plugs?

No.

[scottly]

Jus throwing this out there but could failing CDI units lead to this as well?

No. If anything, the system is producing too much spark, at least for the insulation, but I've never seen such a thing.
Years ago, Chrysler sponsored a trouble-shooting contest for high-school auto shop students, where they would rig cars with problems that had to be repaired. One of the tricks was to take a pencil and draw a conductive path between spark plug terminals on the distributor cap, which was all it took to cause an intermittent miss. Your original problem reminds me of this.
You should be able to cut the lower inch off a cap without any arcing... Is the arcing on 2 and 3 through the side of the cap like it did with the new cap on 4?
 

[papi_sosaa]

Just throwing this out there but could failing CDI units lead to this as well?

No. If anything, the system is producing too much spark, at least for the insulation, but I've never seen such a thing.

What about a hot engine? Could overheating lead to the arcing? However, that wouldn't explain why it jumped from arcing on #4 to #2 and #3.

Is the arcing on 2 and 3 through the side of the cap like it did with the new cap on 4?

Yeah.

Coil 1-4 (CAP-WIRE-COIL-WIRE-CAP): 24.2K Ohms
Coil 2-3 (CAP-WIRE-COIL-WIRE-CAP): 24.9K Ohms

With the plugs, it's right at the 30K Ohms mark and based off what I'm reading on other forum threads, is not great.

Haven't checked JUST the aftermarket coils yet but OEM coils measured at 12K Ohms. The 1982 CB650 calls for resistor caps and plugs which is about 10K Ohms. So spitball, the total resistance should be around 22K Ohms but I'm measuring 24K Ohms without the spark plugs. I'll check tomorrow but maybe it is my cords.

New word of the day: Ohms

[scottly]

Coil 1-4 (CAP-WIRE-COIL-WIRE-CAP): 24.2K Ohms
Coil 2-3 (CAP-WIRE-COIL-WIRE-CAP): 24.9K Ohms

With the plugs, it's right at the 30K Ohms mark and based off what I'm reading on other forum threads, is not great.

5K+5K (caps)+14.9K (coil)+5K+5K (plugs) =29.9K Sounds about right. Older 750s had 10K caps. The secondary resistance isn't that critical, and the added resistance in the caps and/or plugs is to suppress Radio Frequency Interference. 

[papi_sosaa]

Coil 1-4 (CAP-WIRE-COIL-WIRE-CAP): 24.2K Ohms
Coil 2-3 (CAP-WIRE-COIL-WIRE-CAP): 24.9K Ohms

With the plugs, it's right at the 30K Ohms mark and based off what I'm reading on other forum threads, is not great.

5K+5K (caps)+14.9K (coil)+5K+5K (plugs) =29.9K Sounds about right. Older 750s had 10K caps. The secondary resistance isn't that critical, and the added resistance in the caps and/or plugs is to suppress Radio Frequency Interference.

Well, if it's only RF then who cares. Barely anybody listens to the radio anymore.

I'll measure it properly tomorrow and see.

[scottly]

When you first changed the 1&4 caps, it was on the previously greased plugs, correct? You then changed coils and plugs, correct? Is it possible you put the 1-4 caps onto the 2-3 coil leads?

[papi_sosaa]

When you first changed the 1&4 caps, it was on the previously greased plugs, correct? You then changed coils and plugs, correct? Is it possible you put the 1-4 caps onto the 2-3 coil leads?

The caps currently on the bike came in a kit with the coils and cords. NOTHING in the ignition system from spark plugs to the coils have grease on them.

The 2 other caps are sitting on my bench with the 2 original OEM caps.

[scottly]

Maybe you could try cleaning the inside of the oem caps and put them on 2&3? I don't know what else to try..

[MauiK3]

I'm beginning to think an entire new OEM coil, wire, cap assembly is needed

[Don R]

  Maybe I lost track, but did the oem 2-3 caps arc through?
  I'm also wondering why Australia, GB and Germany seem to have NGK caps but none are available in the US?

[bryanj]

GB is getting difficult to find

[rotortiller]

The caps currently on the bike came in a kit with the coils and cords.

Changing the caps would be a good idea. They are sometimes garbage on after market coils which are otherwise fine. One set I bought had no continuity on two caps unless I fiddled with the internal contacts.

[PeWe]

I bought a few sets from UK before Brexit.
They still have but for much more £
More than double today.

https://www.gsparkplug.com/electrical/ignition/spark-plug-caps/resisted-caps

[Deltarider]

This thread is exceptionally long for such a common problem. Is it possible to shoot a video of the engine running? Please zoom in to the plug involved and follow the routing of the HT wires and where they exit the coils. This means tank of.
Personally I have not seen any benifit of 10kOhm resistance to one plug and another 10kOhm to its corresponding plug. IIRC only Canadian models had this. Goldwings also had it. They often were equipped with radio stuff (AM/FM and Citizen Band).

[MauiK3]

Something is being missed

[papi_sosaa]

Just finished doing a bunch of prep and carb cleanup, I'll be able to start it tomorrow to see how it runs.

I took some measurements:
Aftermarket coil: 13.6K Ohms on the secondary
OEM coil: 11.9-12K Ohms on the secondary
Cap with cord was about 5-5.5K Ohms

Made sure all my connections were good, little to no extra resistance where cords screw into cap and coil.

Based on previous comments and 4into1's own recommendations, I've removed the resistor inside the caps and replaced them with brass. Measurements for the modified caps were anywhere from 0.6 to 1.6 Ohms.

I'm also going to reuse the OEM coils. During tomorrows startup, it'll be OEM coils, non-resisted caps, and DR8ES-L spark plugs.
Coil 1-4: 22.5K Ohm (Spark to Spark)
Coil 2-3: 22.2K Ohm (Spark to Spark)

[papi_sosaa]

I couldn't catch it on video but I'm still seeing the slightest little arc on #3. I couldn't see anything on the other 3 caps.

https://gofile.io/d/v11E6U

[papi_sosaa]

I believe I have figured it out. The culprit was the springs inside the cap.

So from the beginning:
After I first noticed #4 was arcing, I bought 2 caps from Vintage750, replaced #1 and #4, and then noticed #4 was still arcing. I bought new DR8ES-L spark plugs and a kit from 4into1 (4 caps, 8ft of 7mm HT cord, 2 coils).

Used the new coils, cords, caps, and plugs, and now #2 and #3 were arcing. Total resistance per coil from plug-to-plug was almost 30K Ohms so I decided I wanted to reuse the OEM coils and remove the resistors from the caps to reduce total resistance.
OEM coils - 11.9-12.1K Ohms
Aftermarket coils - 13.6-14.0K Ohms

I noticed that out of my 6 new caps, 3 of them had different internals; 3 caps had 1 long spring and the other 3 caps had 2 springs. While replacing the resistors with brass, I elected to use only the long springs except for #3.

Total resistance is now 22-22.2K Ohms per coil, plug-to-plug. I fired it up today, no arcing on #1, #2, #4, and a faint arc coming from #3. For cap #3, I removed 1 of the springs and put in a longer piece of brass. No more arcing.

I don't think changing the total resistance would've fixed anything but I believe that extra spring was causing my problem. Maybe the way it was making contact with the other spring, I'm not sure. Where they sit in the cap aligns with where the arcing was happening so I'll take it. I provided some pictures to showcase the difference between OEM resistor vs. aftermarket resistor, as well as the internals difference (replace my brass screw piece with the aftermarket resistor).

I'll keep an eye on it while I finish the rest of my repairs but thanks again to everybody who provided assistance in trying to solve this problem.

[Don R]

 Well that was a pain. Too bad you had to go through all of that grief to make the electrical path to the plug easier than the path through the plug cap to the electrodes.

[scottly]

Aren't non-resistor genuine NGK caps still available?

[papi_sosaa]

Aren't non-resistor genuine NGK caps still available?

So far I haven't found anything.

[scottly]

I noticed in your video your CV carbs were open, and not connected to the stock air-box. A lean mixture between the spark plug electrodes requires a higher voltage before it can jump the gap than a rich mixture, effectively increasing the resistance. Just an FYI...

[papi_sosaa]

I noticed in your video your CV carbs were open, and not connected to the stock air-box. A lean mixture between the spark plug electrodes requires a higher voltage before it can jump the gap than a rich mixture, effectively increasing the resistance. Just an FYI...

My carbs are modified. I installed the Stage 3 Dynojet kit because I'm running pod filters and the main jet is slightly bigger so it runs a little richer than stock.

I had my carbs off in that initial video because I had just cleaned the carbs and I was using carb cleaner to help start it up.