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Pilots,
I took a bunch of old standard plugs and took out the low resistance resisters. I then replaced my high resistance Iridium plug resisters with these low resistance resisters. Here is the before and after resistance.
10/17/10
Top Bottom
1 6.6 m 2.8 m
2 1.5 k 7.7 m
3 4.5 m 2.4 m
4 24.5 k 1.2 k AL
10/18/10
Top Bottom
1 1.68 k 1.18 k
2 1.69 k 1.58 k
3 1.11 k 1.57 k
4 1.22 k 1.05 k AL
In my prior post below, I listed the Autolight (AL) plug in the mega ohm range. This was a mistake. In all of the AL plugs that I measured, the resistance was between 1.1 and 1.3 kilo ohms. AL's do not seem to have resisters that increase in resistance.
After I reinstalled my iridium plugs, the plane started immediately as soon as the key was turned. Before the resister change, the plane took 3, 20 to 40 second turns to start. Over the last 2 years, the plane has been getting harder and harder to start. I put Iridium plugs in about 8 years ago.
From the above charts, I can see that my right side impulse coupled mag which fires the 1 & 3 top and the 2 & 4 bottom plugs was firing only 1 plug, when the plugs were damp. Of course, when trying to start the engine, all of my plugs are damp.
I believe that the resistance takes years to increase. Standard aircraft plugs are changed long before the the resistance starts to increase. My Iridium plugs, although looking like new, have been used for about 1,000 hours and 8 years.
Here are 3 web sights that will help us understand the problem.
1) A 1965 Ft Ruckers, Al study on Iridium plug resistance.
http://www.dtic.mil/cgi-bin/GetTRDoc?Location=U2&doc=GetTRDoc.pdf&AD=ADA030836
2) A 1999 study by Rainier Lamers.
http://www.ultralightnews.com/enginetroublshooting/resistorcapsandplugs.htm
3) And finally a link to the Aircraft Magneto Service shop in Bainbridge Island, Wa. I talked to the owners for quite a while last week and I will be using them in the future for any magneto service that I need. They also reaffirmed that the resistance changes that I have done were appropriate.
http://www.aircraftmagnetoservices.com/
Comments appreciated as always.
Lorn
> I thought that I would measure the ohms on my Iridium plugs today. 7 of the plugs are Champion and the other is Autolight.
>
> 1 top 6.6 mega ohm
> 1 bottom 2.8 mega ohm
> 2 top 1.5 kilo ohm
> 2 bottom 7.7 mega ohm
> 3 top 4.5 mega ohm
> 3 bottom 2.4 mega ohm
> 4 top 24.5 kilo ohm
> 4 bottom 1.2 mega ohm Autolight plug
>
> No, the above readings are not typos or misreads. Just the facts. Something must be wrong with the 800 - 1,200 ohms listed below. Tomorrow I will remeasure with an analog VOM meter.
>
> The plugs all looked pretty good except for the number 2 bottom. It looked very fouled and was not firing quite right from an in flight test.
>
> I think that I may swap the number 2 bottom's resister with one from an old standard plug.
>
> Thoughts are welcomed.
>
>> From: Isaac Heizer <ijheizer@comcast.net>
>> Date: October 10, 2010 10:57:35 AM EDT
>>
>> .
>> .
>> .
>> Then we started talking about spark plugs: I mentioned my plane has Champion fine wire plugs. He said all plugs have an internal resistor, and for some reason the resistance of many Champion plugs seems to increase as the plug ages. Once the plug has been cleaned, the resistance can be measured with a standard ohm meter between the center electrode and the connection inside the plug barrel -- all new plugs should have a resistance somewhere around 800 - 1200 ohms. If the resistance ever measures above 5000 ohms, the plug should not be used..
>> .
>> .
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>> -isaac
>> ES-P N7842K
--
Lorn H. 'Feathers' Olsen, MAA, ASMEL, ASES, Comm, Inst
DynaComm, Corp., 248-345-0500, mailto:lorn@dynacomm.us
LNC2, FB90/92, O-320-D1F, 1,750 hrs, N31161, Y47, SE Michigan
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