Very Weak Spark; 1980 KZ440A1 (with points)

I'm pretty sure I've traced the problem back to the ignition coil. I can get the bike to start, but it sometimes idles very rough. If it idles smooth, then it will start missing badly around 3k RPM (so the bike is difficult/impossible to ride). Here's what I've gone through/checked so far:

1. New battery
2. Checking points gap - It's at 0.015 inch, right where it should be.
3. Cleaning points
4. Checking resistance through the points from where the coil plugs in to the engine case (I get under an Ohm here)
5. Cleaning the connections between the battery and the ignition coil - The killswitch contacts were a bit corroded.
6. Checking the voltage of the red-yellow wire to the ignition coil - Consistently reads about 0.3V lower than battery
7. Replaced spark plug caps - One was an open circuit. After replacement, both read between 4.6K and 4.8K Ohms

Am I missing anything obvious? If I ground a plug against the engine, the spark is really bad (I can't see it, and can only barely hear an arc). If I close the plug gap to about 0.020, then I get an okay spark, but still not really bright.

I did check the resistance through the coil primary and everything *looks* okay there, but I do know that when you're dealing with the multi kV ranges the amount of things you can check with a multi-meter sometimes don't matter. For reference I was getting about 4.8 Ohm on the primary, and 12.2K Ohm on the secondary (with the plug caps removed).

Let's give it a shot.

Which plugs are you using?

You'll need your meter.
meter on DC V
bike ignition on/ engine not running
rotate engine counter-clockwise till points are completely closed.

Black meter probe on negative battery post(the metal part of the battery that the cable attaches to.)
Red meter probe on positive battery post
Note voltage

move your red probe to the positive side of the coil
note voltage
move red probe to negative side of coil
note voltage

If everything is good here, your positive reading will be a few tenths lower than battery voltage.
Your negative reading will be very close to zero. This will be when your coil is charging.

Rotate engine till points are fully open.
black meter probe on negative battery post
red meter probe on positive terminal of coil
note voltage
red meter probe on negative terminal of coil
note voltage
This is your coil firing reading. The coil reading will be the same on both measurements if everything is good.
The readings will be closer to battery voltage than the points closed reading(just by a couple of tenths).

Shut ignition off to prevent coils from overheating

> Which plugs are you using? NGK B7ES

> Black meter probe on negative battery post(the metal part of the battery that the cable attaches to.)
Red meter probe on positive battery post
Note voltage: 12.37

move your red probe to the positive side of the coil
note voltage: 11.07 Note: this is with the coil plugged in drawing current. With the coil unplugged, this terminal gave me 12.20V)
move red probe to negative side of coil
note voltage: 0.27

Rotate engine till points are fully open.
black meter probe on negative battery post
red meter probe on positive terminal of coil
note voltage: 12.20
red meter probe on negative terminal of coil
note voltage: 12.20

Your voltages look ok for your coils and your points.
Granted, I would like to see a higher positive voltage at your coil, but even with this voltage at the coil you shouldn't be having the issues you are experiencing.
We may direct you to the coil relay modification to get your coil voltage closer to the battery voltage of 12.37.

What do your spark plugs' insulators look like(black, brown, white...)?

Run your point gap to .0118in or .30mm.
You'll be tightening the gap so the points stay closed longer
This will give your coil more time to build a charge.
Re-check your timing.

I have had problems in the past with grounding plugs to the block.
Sometimes they spark. Sometimes they don't.
I also have two spark plug testers that give me varied results.
A good clean ground is necessary. So I'm not sure how much weight you could put on your spark test.

Do a meter reading for me
I want to check if the cylinder head is well grounded.
If the ground is bad on the cylinder head then that could inhibit the spark quality.

Bike ignition on/ engine off
Spark plugs installed
meter DC V
red meter probe on negative battery post
Black meter probe on the hex nut of the spark plug.

This is a voltage drop reading
If the voltage is higher than .5 V then there is too much resistance between the battery and the cylinder head.
Your spark plug needs a good ground for a proper spark.

Hmm, I got 0.02V on the right plug, and 0.01V on the left. So that's probably not the problem.

I'll re-gap the plugs to the proper gap tomorrow and try setting the points gap. The dwell is currently about 196°, but I'll try increasing that to get a bigger charge on the coil. They are still pretty new, but they look about right (light tan colouration, but the insulator is still a bit shiny. These plugs have less than 100 mi on them.

There's some arcing on the points going on right now. I replaced the condenser last month, so I don't think that's failed already.

cylinder head ground is very good.
Your mixture is fine. I saw you rebuilt the carbs in an earlier post.

Your ignition system is fairly simple.
battery,coil, plugs and a switch(points).
So not a lot to fail.

You may be correct on the coil.

Does the spark go bad after the bike warms up?

When the problem first occurred, it ran great when I first started the bike up that day, and gradually got worse as the engine warmed up. I'd also noticed slight, occasional misfires at speed on the highway (65-ish MPH) after five or so miles of cruising. It would be fine running all day on regular streets. When it failed big time, it was better until it got warm, at which point it wouldn't even really idle well. Closing the spark plug gap helped that a bit.

I did get a used coil as a replacement (I believe it came off of a KZ750 Twin with points). It was the same part number as my coil And electrically seemed to be identical. After that replacement (which is when I replaced the plugs), it ran great for probably 40 miles or so before that one failed too, this time leaking dielectric oil out of the casing (so an obvious coil failure). After that I put the original coil back on and it got a bit better, but not fixed. That's another thing that leads me to believe it's the coil.

The new coil should arrive tomorrow. I'll throw it on there and see if things are improved, and if not I'll have a look at the points gap and see if that helps any. I mostly wanted to make sure I wasn't going down a wild goose chase and there was something stilly that I was missing. Sounds like I've covered pretty much everything, and if changing the coil changes things / occasionally makes it work well, then that's probably the issue.

EDIT: I did also try and check the resistance between my cylinder head and the battery (as a check for a bad ground) but it was giving me 0.2 Ohm, which is the reading I get if I short the probes together. So it looks like that is low enough that I can't even measure it with my multimeter. The voltage drop test was a good idea.

Yeah, it seems like a coil failure. The hot failure is a clue as well. The other parts don't really have heat failure issue.
There always can be some freak problem but for the most part you covered everything.
The gap will help some but I really don't think it will fix it.
Put on the new coil first.
Checking resistance with the ohm setting is not the best.
Voltage drop is better.
The meter will give .1 ohms even though there is only one tiny strand left on the wire.
With voltage drop the meter will show the restriction.

Well, USPS has apparently lost my coil, so maybe I'll be able to replace it.

Uggh, Hopefully it turns up.

The new coil finally came. Before installing it, I decided to check the winding resistances. The primary is at 4 Ohm, so that's about perfect (I confirmed this with the seller before I bought it). However, the secondary has about 17 kOhm resistance. The spec says about 10-15. Is this going to be a problem?

Give it a shot.
Im guessing the secondary may have turns than the stock or smaller wire.