Ignition Circuit resistance

[Barry]

I don't have a current problem with my ignition but for the sake of creating some traffic on the forum I'm posting about a problem I had many years back.

My points ignition has always worked but I did have some concerns with the strength of the spark.  To test the spark, I opened up the gaps on some old plugs to 80 thou, laid the plugs on the heads and cranked the engine.  The points ignition had no trouble sparking across that gap but the spark was as much yellow as is was blue.

To check out the ignition system I did the standard ohms measurement test from one plug cap to the other. What you are supposed to read is the secondary resistance of both coils plus the resistance of the plug caps.  The previous owner had replaced the original 1k caps with NGK 5k caps so what I should have read was 7.5K + 7.5K for the coils plus 5k + 5k for the caps giving a total of 25K ohms.

What i actually read was infinity and yet the bike seemed to run fine albeit with some yellow in the spark.

Testing the individual components soon showed the coils to be ok at 7.5 K each, the HT lead was solid copper and essentially zero ohms but the caps both read open circuit.

I resolved to buy new caps but thought I'd investigate first.  The NGK caps have a ceramic resistor and a spring held in by the termination that screws into the HT lead. If you grab that termination that looks like a wood screw with needle nose pliers and yank, it will come out along with the resistor and tiny spring. (Not a bad idea to do that inside a large plastic bag if you don't want to grovel on the garage floor looking for the tiny spring that flew out).

I found the open circuit was caused by corrosion on the end of the termination. I guess the spark was able to arc through the corrosion in use so the the bike still ran even though it read open circuit with a ohm meter.  The termination was secured by a couple of dished star washers so with everything cleaned up and pressed back into place, the caps measured the 5K that they were supposed to do. 

The spark was now noticeably bluer and fatter so it was a worthwhile exercise.  I never did buy new caps for the simple reason that I couldn't find the correct for points 1K caps in the UK. No problem finding zero ohm caps or 5K caps but nobody sells 1k Caps.  The nearest I came was 1K Beru metal clad caps but I didn't fancy those.  I've only ever seen NGK 1K caps on US web sites.

Anyway the learning point was it's possible to dismantle NGK caps and reassemble them.

[georgesgiralt]

Hello Barry,
What was true then may not be so now. A couple of years back two of my BMW spark plug cables went south. One was found cut at the cap end (insulation destroyed by too strong the pressure by the cap) and the other one open circuit and no spark on the plug. So I bought NGK 5K caps (electronic ignition) and thought I was happy as a clam. Alas, at first rain, the bike started to misfire. So I thought it was somewhat corrosion related as the caps where new. Alas impossible to remove anything from the cable end. or the plug end. Everything was encased by some sort of varnish/glue and the story ended with a broken NGK cap. Both caps lasted for less than 1000 km. So I asked the banker for an extension on the mortgage and bought two BMW OEM spark plug caps and returned riding again... Since that time, I keep myself away from NGK products.
As my grand mother used to say "we are not rich enough to buy something cheap".

[Barry]

My NGK caps are a minimum of 15 years old so they must have changed the production method.  I never did have a problem in wet weather though and the bike was ridden for commuting right through the winter before I retired.

I'll have to start looking again for 1K caps. This is the only one I ever been able to find,  the Beru metal clad type similar to BMW originals but even these are not stocked in the UK.

[Bob_Roller]

I had an interesting ignition system problem about 20 years ago .
When the temperature dropped below 55 F, 13 C the engine was hard to start first cold start of the day .
The fuel supply had just changed from MTBE to 10% ethanol a few months earlier in warm weather .
I blamed the ethanol on this .
To make a long story short, the primary coil circuit had .7 ohms should be 1.5 ohms, OEM dual output coil, second generation the red and black coil .
The multimeter I had showed the correct value, I took the coil to work and used a calibrated meter, that's when I found the low resistance .
After that, I bought a Fluke multimeter .
I went to Dyna brown coils on all the bikes, no issue with them yet .
I've used NGK ignition leads for 20 years, no issues with them .

[Barry]

Cheap multimeters are remarkably accurate on voltage measurement and more than good enough for our purposes.  Measuring low resistance values accurately is much more challenging and unlike the old analogue meters, cheap digital meters have no means of zeroing the display when the leads are shorted. Flukes are the business when you want all round accuracy.