Ignition Coil Testing Nitty Gritty

[bcware]

Since I am asking a silly question I decided to go all out and include photographs. Perhaps this will aid someone else at some point.

I am testing my old ignition coils for giggles, but I have a question the Hayne's manual was not able to answer. The Hayne's manual says to remove the spark plug cap from the spark plug, but it does not mention removing the spark plug CAP from the cable.

The primary coil tests normal: approximately 4 ohms.

When I test the secondary coil with the cap on I get readings that are too high; i.e. 13.5 kohms to 14.5 kohms.

With the cap removed I get readings that are in spec; i.e. 9.28 kohms to 9.4 kohms.

"Normal" readings are as follows according to the Hayne's manual:

Primary coil resistance: 4.0 ohms +/- 10% at 20 degrees Celsius
Secondary coil resistance: 9.5K ohms +/- 20% at 20 degrees Celsius

For the newbies:

The photos demonstrate how to test the coils, but keep in mind that they need to be held in place firmly. If you're not making good contact with a cable/wire the readings will fluctuate wildly.

Select the correct scale on the multimeter for each respective cable/wire. If the scale is wrong you'll probably get a 1 or a -1. I use a 20 ohm scale for the primary coil and a 20k scale for the secondary coil.

SO, the question remains: should the spark plug cap be removed? Alternatively, are my spark plug caps bad? Did I twist them on wrong?

 

[xschris]

You do the tests without the cap.

 

[bcware]

Do you have any idea what kind of resistance the caps should have? Does it matter? I measured mine and got something like 3.28 kohm and 3.8 kohms.

When looking this up there seems to be a lot of argument over whether resistor caps or non resistor caps should be used with point assemblies. I haven't had much luck finding a good explanation of how the caps normally function and their anatomy. I realize a "resistor" cap has a resistor in it, and a non-resistor type lacks this resistor, but does that mean a non-resistor type cap offers very little resistance (offering none seems impossible)?

From what I have found so far bikes with points normally have non-resistor caps. Bikes with a cdi box use a resistor type.

 

[xschris]

I use non resister caps for both of my bikes points and tci. But I use a resister plug ngk bpr7EIX which has a resistance of 5k ohms. More resistance will give you a shorter spark duration where less will gives you longer so I have read. All I know my bikes run much better with this set up. I think the stock caps where 7-9kohms with a non-resister plug. Your setup should have at least 5k ohms of resistance or it could damage the tci or over heat and or wear out plugs, so I have read also.

 

[Drewcifer]

They are resistor caps to prevent (I'm not kidding) RF frequency interference. You don't need them. I ordered replacements that took way too long to arrive. Mikes XS has them too. I think I have a the whole deal, wires, caps, condenser and coils. Turned out I only needed new caps.

Good luck.

 

[bcware]

One of my caps definitely tests a few hundred ohms lower than the other, but both are only giving around 3k ohms. Should I just replace them? Can I get something like this locally?

I have points ignition, so damaging the tci is not an issue. Perhaps the cap(s) are also contributing to my heat problems on one side.

 

[xschris]

Go 0ohm caps with the iridium plugs If it helps I think drewpy uses them too If your resistance is only 3ohms with a non-resistor plug that is why your plugs are so hot.

 

[chkm8]

If your plugs are running hotter, would that also explain why the engine appears to run lean?

 

[bcware]

Yes, leaner mixtures run hotter and are therefore much worse for the health of the bike. If you're too rich you'll just foul things up, but cleaning fixes that.

The new coils and wires are installed; I'm going to do a test run. If all goes well I will hit the auto parts store and get some bpr7EIX plugs.

Where is the best place to get the caps?

 

[chkm8]

That's what I was wondering. If you're running hotter plugs that might explain why your bike seems to always run lean. I think my logic is right, I haven't had coffee today

 

[xschris]

Yes, leaner mixtures run hotter and are therefore much worse for the health of the bike. If you're too rich you'll just foul things up, but cleaning fixes that.

The new coils and wires are installed; I'm going to do a test run. If all goes well I will hit the auto parts store and get some bpr7EIX plugs.

Where is the best place to get the caps?

Ebay has them, mikesxs you can even get just the rubber caps that go on lawn mowers. This is what I have on my 80.

 

[bcware]

Update:

New coils and xs performance wires with old caps and new bp7es plugs on both sides.

I did about 35 miles and there was no change in performance. There is still an intermittent miss at idle and the right side pops more and more on decel as the bike gets up to full temperature. I did not exceed 8,000 rpms throughout the ride. It was mixed stop and go with 55 mph stretches.

Currently I am waiting for the bike to cool down so I can take a look at the plugs. While I was out I ordered some bpr7eix plugs and picked up some non-resistor boots and terminals (new terminal shows only about 0.5 ohm resistance).

When I put the new coils and wires on I made sure to swap the plug caps around; good side went to the problem side and vice versa. Since it had no affect on performance I am leaning more and more towards the sleeve insert as the problem.

 

[XSLeo]

Nice pics, should help people a lot.
With 3k+ ohm caps, they were probably O or 1k ohm caps when new, As caps age the ohms get higher. I had some old 5k caps that tested closer to 8-9 k ohms. Bike ran ok but with new caps it ran better.
On the miss and pops on decell, that sounds like carbs to me. Maybe air leaks in the exhaust.
Leo

 

[xs400neophite]

I think I got the same issue but plug sleeves all original.
Its gotta be a timing/ignition/battery thing?

 

[bcware]

I am considering the exhaust leak carefully. After all, I installed these mufflers without gaskets; I used ultra copper silicone and clamps. This was a crappy method of installation, but my options and funds were limited at the time. I am looking at replacement systems now.

 

[XSLeo]

If your mufflers are 1 3/4 inch inlets, go to the Harley dealer and get their clamps. They are about the best clamps I have found. It would be nice if some one could make clamps like that in other sizes.
I don't know the part number or price, but I think they are less than $10 each. Well worth the money, they were the only ones I tried that would seal my mufflers to the pipes. Here's a pic.
Leo

 

[Yellow0206]

This is an old thread but a good one re coils - however you say “use 20ohm and 20k ohm setting and there is not a 20ohm on your multimeter (i have the same one)

 

[NewHavenMike]

You can use almost any ohm setting, the only thing that changes is the decimal point. It can be confusing reading .005 ohms on a 20k scale when it would read something like 5ohms on a 200ohm scale.

obviously it requires some math to figure the conversion, my numbers are not accurate but it should give you an idea

 

[Yellow0206]

Thks Mike - per other thread still trying to get rid of my high rpm miss and am thinking now it almost has to be coils and or wires

 

[xschris]

Test coils without wires and caps in them. The 77-79 coils have non-removable wire but are non-resistor copper core so that wont matter. The 80-82 coils have removable copper core wires. If the wires are worn any auto parts store should have 7mm copper core wire in bulk rolls for sale. I use stuff that has silicone around the copper core. If you have non-resistor plugs you need 5k ohm resistor caps. If you use the iridium resistor plugs you need a non-resistor cap.