Topic: cb550 kills NGK Spark Plug caps.

[xhevi]

hello all.

questions:
1) has anyone had the NGK caps burn out in few hundred miles?
2) what can cause a spark plug cap to burn out?

info:

1976 CB550, 12K+ miles, stock air box, 4into1 exhaust. NGK D7EA plugs

last summer i replaced the spark plug caps with NGK 5 ohm from 4into1. it ran fine for a couple hundred miles then started running rough.

i spent hours looking into the carbs, wiring, compression and everything else and could not find an issue. last i measured the spark plug caps to find that one was reading into mega ohm resistance. i do not remember if it was cylinder #2 or #3. the other three caps were fine. i bought 2 new caps for the middle cylinders (on Amazon this time), one to install and one as a spare. and it was running much better.

this spring, a couple of hundred miles later, it started running rough again. and AGAIN i spent hours looking into the carbs, wiring, compression and everything else. including:
 
1) tightened all spark plug caps and looked for arching of the plug wires into the engine frame (so damn close to the culprit): all looked good
2) installed Dyna S ignition. no help
3) cleaned and checked the advance mechanism: no help.
4) back to points ignition, set point gap and timing: no help
5) adjusted cam chain tension and valve clearances: no help
6) few rounds of vacuum synching carbs: no help. i also had a running rich issues (all 4 plugs were fouling) while it was running good. i believe i solved that by dropping the needle from 4 clip from the top to 2nd clip from the top.

for the SECOND time i forgot to check the coils and spark plug caps and paid dearly in time and money.

frustrated, i gave up fussing with old parts and ordered new kiehin jets and needles, and also got a new set of 5Ohm coils that come with wire and new plug caps from 4into1.

today, i am setting up to install the new coils and measured the new coils and caps ohms, and all is good.

measuring the old coils (from boot to boot) i got coil for #1 and #4 to be around 25kOhm (good) and coil for #2 and #3 open. took the spark caps off and on both coils, primary and secondary check out at about 5ohm and 15kOhm. so the old coils are good. checked the plug caps individually and found one to be shot (either #2 or #3)

i learned my lesson, next time it runs rough i will  start with the plug caps. so much wasted time, money but above all how frustrating it can get. 

so now i need to find out why this happens.

i could keep the old coils and replace the burned cap and hope for the best, or just install the new coils and still hope for the best.

thank you.

[bryanj]

Excess vibration or heat will do it and its 1 out of 4, a high average for you but that could be 1 out of a batch of 100+

[TwoTired]

The resistor in many plug caps are held in place by a screw mechanism to hold them tight.  If they aren't tight, there forms a gap.  A small gap will arc the same time as the spark gap in the plug and is immediately uneventful.  However, the spark jump causes erosion making the gap ever wider.   This causes the voltage to rise higher before the spark initiates jump, but when it rises it also increases the current during the spark event and that advances the erosion ever faster, until the accumulated gaps become large enough that the reserve potential stored in the coils becomes iffy for initiating the spark in all the accumulated gaps in the HV circuit.  That would be both spark plugs, and both the plug caps with internal gaps, as well and any other loose connections in the pathway.

In theory, if you can dismantle the plug caps and eliminate the gaps, with new resistors, the cap will be restored to new operation.  If the existing resistor has not burned too much and you can eliminate the gaps with additional metal to adjust them for full contact, they will again function as required.  Anyway, the key to long life of the cap is to not have any gaps occur inside the plug cap.

I speculate the volume of plug caps now being made is now very low, and to make more profit the quality control testing by manufacturer has been reduced or eliminated.  So, test measure your new caps for for resistance.  If open circuit when new, they will certainly fail earlier than one that was made with intact contacts internally.

It's also possible that new formulations of compressed carbon used for the resistors are now more porous, which would make tiny erosive arcs and heating occur within the resistor element itself, again making the resistive element slowly destroy itself.

Finally the captive mechanism may be crushing, or putting too much pressure on the resistive carbon element, and with additional vibration, it crumbles. These parts are all made with tolerances, and it is cheaper to make parts with wider tolerances.  However, when all the tolerances stack the wrong way, that's what quality control measures are supposed to catch by doing random recurrent sampling of the final product, if such measures are deemed to reduce recalls from part failure.  But, when the company determines their is no monetary benefit from QA, such programs are terminated.

However, without an investigative analysis of the failed parts, the exact cause may not become known.  Kinda have to be there, if you know what I mean.

Cheers,

[xhevi]

The resistor in many plug caps are held in place by a screw mechanism to hold them tight.  If they aren't tight, there forms a gap.  A small gap will arc the same time as the spark gap in the plug and is immediately uneventful.  However, the spark jump causes erosion making the gap ever wider.   This causes the voltage to rise higher before the spark initiates jump, but when it rises it also increases the current during the spark event and that advances the erosion ever faster, until the accumulated gaps become large enough that the reserve potential stored in the coils becomes iffy for initiating the spark in all the accumulated gaps in the HV circuit.  That would be both spark plugs, and both the plug caps with internal gaps, as well and any other loose connections in the pathway.

In theory, if you can dismantle the plug caps and eliminate the gaps, with new resistors, the cap will be restored to new operation.  If the existing resistor has not burned too much and you can eliminate the gaps with additional metal to adjust them for full contact, they will again function as required.  Anyway, the key to long life of the cap is to not have any gaps occur inside the plug cap.

I speculate the volume of plug caps now being made is now very low, and to make more profit the quality control testing by manufacturer has been reduced or eliminated.  So, test measure your new caps for for resistance.  If open circuit when new, they will certainly fail earlier than one that was made with intact contacts internally.

It's also possible that new formulations of compressed carbon used for the resistors are now more porous, which would make tiny erosive arcs and heating occur within the resistor element itself, again making the resistive element slowly destroy itself.

Finally the captive mechanism may be crushing, or putting too much pressure on the resistive carbon element, and with additional vibration, it crumbles. These parts are all made with tolerances, and it is cheaper to make parts with wider tolerances.  However, when all the tolerances stack the wrong way, that's what quality control measures are supposed to catch by doing random recurrent sampling of the final product, if such measures are deemed to reduce recalls from part failure.  But, when the company determines their is no monetary benefit from QA, such programs are terminated.

However, without an investigative analysis of the failed parts, the exact cause may not become known.  Kinda have to be there, if you know what I mean.

Cheers,

thank you Twotired.

by captive mechanism do you mean the end with "threads" where the spark plug tip is connected? and by pressure do you mean if i pushed them down onto the spark plugs too hard?

i have gotten in the habit of measuring new parts when i can to make sure they are ok before i waste time installing them. both times the caps checked out around 5kOhm and worked ok for sometime. both failures occurred at the same coil, #2 and #3 coil, was wondering if there is correlation there and hopefully i can address it so i does not happen again.

yes the caps are relatively cheap compared to the rest of the parts, but i feel like have spent 100s of hours reading up and in the garage and only a few hours actually riding it.

working on this bike has been fun, just not as much fun anymore

[Flyin900]

The NGK caps are 5K ohms not 5 ohms as you originally posted. Usually the coils are a few ohms on the 12V input side and depending on the coils they are typically 3-5 K ohms on the spark plug output side per wire. So when measured through the two spark plug leads and two NGK plug caps you should be reading anywhere from 15-20 K ohms of resistance.

Are you using new spark plug wires with all these new caps, or if they are fixed into the coils permanently are you cutting back some of the old ends of the wire to get fresh wire into these caps?

I have had new caps fail, as there is a 5K ohm resistor inside the cap that does fail from time to time and can sometimes be accessed by unscrewing the slotted brass nub inside the cap where the spark plug top metal piece inserts into the cap. These resistors do fail, yet normally on older plug caps, although I have had some new ones fail after a year or so for whatever reason.

If yours are failing very quickly it is hard to determine what is happening ,yet it could be a loose internal connection to the resistor slug piece inside, or possibly a bad batch of caps.

As suggested check every new/old cap for 5K ohms resistance. Also check as you did the pathway through the high tension side of the coils with the new caps installed for the correct 15-20K ohm spec.

[BenelliSEI]

Just add to the misery..... NGK has reported cheap, fake copies of their caps being sold around the world.

[xhevi]

Missed a few 0s there. All caps were about 5000 ohm not 5. To be specific, 10 New NGKs I have measured were between 4.5 to 4.8 Kohm, only one was at 5.2.

I clipped the wires before installing the NGK caps. The coils are oem and wires can not easily be replaced (I know there is a way to cut or drill into the coil casing and epoxy new wires in.

I can not recall where I got the 24 Kohm number from one boot to the other. But I know it's been in a few threads.

I got a new set of Manga 5k coils from 4into1 which came at about 17 Kohm without caps. If I add the both caps it should read 25 to 27 Kohm.

Any feedback wrt these sets of new coils from 4into1.

I Have to decide if I keep the old coils+wires and replace the bad caps....  or just go ahead to install the new coils with new wires and caps

Cheers

[bryanj]

Coils rarely go bad, you US guys seem to have a fixation about changing them

[Deltarider]

+1. The only coil I had go bad, was a Dyna,

[BenelliSEI]

+1. The only coil I had go bad, was a Dyna,

#1 to this. Only coil I have ever had that was a problem was some aftermarket part.

[TwoTired]

Only problem I had with original coils was at the join to the HT lead.  Probably caused by me bending it too close to the coil.  Though I don’t remember doing that.  No doubt the coil itself is still good.  But, I can’t find aluminum strand core wire to do a restore effort.

Cheers,

[xhevi]

to check for any breakage in the wires themselves i hooked up a digital ohm meter to my coils and tugged and pushed and twisted the plug wires any other way and the reading was steady about 14.6 Kohm +/-. the sheath looks ok, not dry or cracked. not sure if that remains the same when the wires are get heated while riding.

as TwoTired mentioned it, it seem that the weak point was right at the end of the sheath where it meets the coil. as i twisting the wire it looked like it was also twisting inside the coil.

i take it that my coils and wires are good. good news but also "bad" news. thought i was done with it, but i got to keep digging. 

i also remeasured the old caps. reading starts at 0 and slowly builds up to 4.5 Kohm. (even the one i called toast yesterday)

i also found that one wire of the meter leads was kinda frayed. will have to dig out my analog meter.

[Deltarider]

to check for any breakage in the wires themselves i hooked up a digital ohm meter to my coils and tugged and pushed and twisted the plug wires any other way and the reading was steady about 14.6 Kohm +/-.

That's the way to check it. Some may judge their OEM coils have become flimsy, just because the wires move so easily near the coils. Most will still prove OK however. Mine always have been like that and I have stopped worrying about it long time ago.
Your cap problem is really odd. Verify once more caps are well connected and see to it there is no accidental arcing between wires/caps and the head. This is best detected in the dark. Your measurements seem fine: coils primary should read 4,5 - 5 Ω, coils secundary (via HT leads and caps removed): 13-15 kΩ. With caps: some 8-15 kΩ more. Can't rule out you had bad luck and someone traded you counterfeit ones.
Your story, how sad it may be, demonstrates once more that, whatever the issue: always check and doublecheck the ignition first. All aspects.

[TwoTired]

by captive mechanism do you mean the end with "threads" where the spark plug tip is connected? and by pressure do you mean if i pushed them down onto the spark plugs too hard?

My spares are in storage, so this is from about 10 year old memory. 

Ref: the plug cap where it goes onto the spark plug.  Deep inside where the spark lead threads go, you'll see two slots straddling where the plug connection goes.  With a proper screwdriver to fit, this part unscrews and behind it is the resistor carbon.  Used to be NGK offered the plug cap in different resistance options and this was how they made the part different for those markets, by using different carbon.  I've not seen more modern part offerings.  Maybe they did a design change.

There is no way pushing the plug cap too hard on to the spark plug should damage anything.

Cheers,

[xhevi]

here is a photo of the guts of an NGK cap.



this one is for cylinder 1 and 4.

on the smaller ones for #2 and #3 i do not see a slot for the screw driver.

anyway. thought i would share this if anyone is curious.

thank you for your input.