Plug caps

[wilcom]

I looked at the plug caps on my 1984LS and they said 1K ohm.
"Is that right", I'm thinking. I looked in a box for a spare wire and cap that I had for my 1979 and it was a 5K ohm cap. This was opposite of my thinking. I thought the points got 1K and the electronic ignition got the 5K Cap.

I searched here and I did not come up with what I was looking for. I went to Snow Bum's site and there was just too too many words to digest.

assuming non resister plugs

option 1,,,,,,Points gets 1K caps and Electronic gets 5 K caps
option 2,,,,,,Points gets 5k caps and Electronic gets 1 K caps

Which option is correct?

[Justin B.]

I thought both were 5k.  I'll check in my junk-box and see what I used...

[wilcom]

I searched here and I did not come up with what I was looking for.

I did some more searching on here and sure enough I even said use 5K cap for electronic about a week ago here on the forum,,,,,,,,,, and then I read where Barry has 5K caps and I bellieve his is still points..........

I'm really going crazy now LOLOLOL

[Tony Smith]

Don't rush out and buy new caps if the ones you have are in good condition - just buy the resistor version of the plugs and make a big mental note of the fact.

[wilcom]

Don't rush out and buy new caps if the ones you have are in good condition - just buy the resistor version of the plugs and make a big mental note of the fact.

Tony ... was that a vote for option 1, 2 or a (3) maybe....?

[wilcom]

I searched here and I did not come up with what I was looking for.

I did some more searching on here and sure enough I even said use 5K cap for electronic about a week ago here on the forum,,,,,,,,,, and then I read where Barry has 5K caps and I bellieve his is still points..........

I'm really going crazy now LOLOLOL

I found it on the airhead site. and I quote,

"On the early Airhead points type models, spark plug caps contained a resistor of 1000 or 1200 ohms."
" Later Airhead points models and all electronic ignition models used 5000 ohm caps for even lower RFI, with added safety benefit for the 1981+ electronics ignition..."

So the R65 point models are "later" then they get 5000 ohm caps just like the electronic ignition models and Justin was right , they both are 5000 ohms...... Mine just has the wrong caps.

[Barry]

I did some more searching on here and sure enough I even said use 5K cap for electronic about a week ago here on the forum,,,,,,,,,, and then I read where Barry has 5K caps and I bellieve his is still points..........

I'm only using 5K caps because 1K NGK caps are not available in the UK. I don't know why not as they are available elsewhere. 

There is a small spark energy loss using 5k caps on points ignition. While it  doesn't normally affect running it does remove some of the safety margin so  I would prefer 1 K if I could get them.

[Justin B.]

I checked my "stash" and they are all 5k - all of our bikes are the later non-points bean-can models.

I know that in theory you should be able to use a non-resistor cap and resistor plugs.  Has anybody done this without bean-can or ignition amplifier issues?  The non-resistor plugs seem to get harder to find as the years go by so this might be something we might need to do in the future...

[Bob_Roller]

What is the resistance of the plugs ??

I've never put  meter on  spark plug .

I did a little searching this morning about spark plug resistance .

Looks like resistor type plugs fall in to a range of 5,000 ohms, plus or minus 200 ohms or so, seems to be an industry standard for general use automotive plugs .

I can't see why this would cause any issues with the ignition system .

There is a ceramic resistor inside the plug and it can change it's value with use, mishandling .

A value above 7,000 ohms can start to effect engine running .

Looked for the plugs I just removed from the oilhead, they went out in the trash on Friday .

[Barry]

5K Ohms is insignificant compared to the nearly infinite resistance of the spark plug gap so a spark plug will fire almost regardless of the amount of additional resistance in the circuit. Until it does fire no current flows and the 5K isn't limiting anything.
 
Once the plug fires the resistance of the ionised gas in the spark gap is much less than infinity and the 5K  resistance becomes relevant.  Because the coil has a set amount of energy to deliver it now can't deliver it as fast which has two effects, a reduction in current flow in the spark and an increase in spark duration.  What you get is a longer but slightly weaker spark.  Longer spark duration is a good thing and the resistance value in any particular ignition system may have be designed with spark duration in mind.  Ohms law means a weaker spark is inevitable as some energy is dissipated in the 5K resistor. Whether it has any practical impact on the ignition system is the only thing up for debate.
All it normally does is reduce the safety margin that the ignition system has. On a well tuned engine a 5K resistor goes unnoticed even with points ignition but that doesn't make it optimum if the system was originally designed for 1K ohms.

Some where I have write up of bench tests done by an electronics engineer. He found that even progressively larger values of resistance all the way up to 1M ohm did not prevent a spark plug from firing but it reduced the safety margin to such an extent that if you so much as breathed on the plug it would not fire at all. 

[Semper Gumby]

Yes.  Points set up needs a minimum of 1000 ohms in the line between the coil and the cylinder head.

EI systems need 5000 ohms.  How you get that amount of resistance is up to you.

My R65 currently has points and the NGK 1000 ohm caps.  The plugs are NGK Iridium BPR8EIX.  The coil is a dyna 3.0 ohm.  So I am running too much resistance.  But *Surprise* the bike runs just fine and starts in the dead of winter no problems.

BTW NGK resister plugs usually run about 4300 ohms so on a EI bike you can use the 1000 ohm caps and resistor plugs and get 5300 total ohms which meets the 5000 ohm requirement.  300 extra ohms is just not that critical.