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Spark plug cap impedance 13 Mar 2007 17:47 #119903

  • KaZooCruiser
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I searched the factory manual, the forum, and found not a clue.

There is quite a lot of discussion about what resistance is correct for coils, but it occurred to me that sometimes spark plug caps can cause problems of their own.

I thought it would be a good idea to check mine, and tried to do so to two of them, and took them off the bike. 3 and 4 both showed open, no matter which scale I used on my digital multimeter.

I tested the meter and leads, and the meter dropped to .04 on the 200 ohm scale, so I know at least part of the testing circuit is working. Off a little, but functional.

I don't think any of the bikes use a resistor plug, and I don't, and I don't seem to have any real issues, starting, idling, etc., but I just couldn't get any reading, hence, no knowledge as to whether 29 year-old-parts are fine, marginal, or out of spec.

Thanks in advance, oh great wise counsel.

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Spark plug cap impedance 13 Mar 2007 17:58 #119906

  • Duck
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5K Ohm is pretty common for use with non-resistor wires.
NGK caps are only a few bucks each for new.
-Duck

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Spark plug cap impedance 15 Mar 2007 12:00 #120427

  • KaZooCruiser
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Well, as usual, it isn't the equipment, it's the operator. Thanks to all that replied. Duck, you got me pointed in the right direction. Good to see you are recovering with all of your reconstruction.

When I finally got my meter set correctly, to a 20k scale, and actually made contact with the bitty contact inside of the cap, I got the following readings, not necesarily on any particular cylinder.

4.17
4.62
4.63
4.74

I ended up putting the 4.17 and 4.74 caps on one of the coils, and putting the other two on the other. In poking around at some of the aftermarket sources for these things, my handy dandy meter showed a reading on the new caps of 5.3k or thereabouts. NGK lists their caps at 5k, but the couple I checked were all higher, so, I guess I wonder what effects are with what I have and what I could expect if I found 4 of these things matched at the same value (not likely) versus what I have, which as I indicated, are not giving any issues that I am aware of. If a $12.00 performance increase is available, I'll rush right out and invest it. But if it isn't, $12.00 still buys a lot of fuel. What say all?

Thanks in advance.

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Spark plug cap impedance 15 Mar 2007 13:41 #120454

  • steell
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Matching resistance in the plug caps is not critical, as long as it's less than 10k or so you probably will not be able to detect any difference in performance.

Spend that $12 on fuel and ride :)
KD9JUR

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