What are we sourcing for replacement ignition coils for the MX / SC machines? I've seen some using the Amazon models. I would like to stay OE or high energy aftermarket. Thanks.

I've rarely come across a bad ignition coil...  only ones I can think of had physical damage and/or showed an open circuit with a meter...  yet many people replace them and if they are having a spark problem it doesn't fix it.

Aftermarket is a crap shoot, though many (including myself) have had luck with an Emgo universal coil I've posted here several times.  I'm not aware of a "high energy" aftermarket coil from a reputable company that is made now or back when these bikes were new...  probably because it's not needed, stock coil is excellent. 

Several used stock coils on ebay or I've known people to use this new Yamaha coil on many bikes  www.partzilla.com/product/yamaha/355-82310-40-00

Thanks for that message.
The Yamaha shop manuals state .7 ohms primary and 6000 ohms secondary windings. Plus or minus 10%.
One manual states remove the spark plug boot as it is a 5000 ohm resistor cap. One manual does not state remove.
Any way, my coil measures out at .8 ohms and 6500 ohms. WITHOUT CAP. A bit over the threshold.
I have not checked for spark. I also have never tested coils. I'm sure there are coils far 'worse' than mine and working fine.
I will proceed though with this coil in my resurrection efforts and update with results.
Thanks guys...

What temperature was the coil when you measured the resistance?  The general "Yamaha" rule is to take the measurement when the part itself has been stored at a stable 68 degrees F... 

Digital or analog meter?   Zeroed before measuring?  If using a digital meter, did it have/was it using "relative" mode where the resistance of the leads is subtracted from the reading?

My point is that there are a lot of variables...  in my personal experience I have never had a bad ignition coil indicated by resistance being off slightly. 

Very good questions.
I measured at an ambient temp of 60 F.
I used a Fluke 117 digital meter and I can't find if this unit subtracts the leads in it's measurements.
www.fluke.com/en-us/product/electrical-t...ltimeters/fluke-117#
The leads I'm using measure out at a .3 ohm.
So a device that should measure .61 ohms +- 10% is within an error window of being inaccurate if the leads are captured.
I feel this coil is within Yamaha's specifications assuming we subtract the leads resistance in the equation.

Actually I had a typo that I fixed a couple hours ago.  68 degrees F, not 60.

Still doesn't matter though...  I think you're fine in terms of resistance readings.  A Yamaha Electro-tester has a circuit that uses a battery to check the coil for actual spark if I remember correctly...  it is possible to have a coil that has proper resistance and bad internal insulation that will only fail under "load" (or if you prefer, "spark").  Not very common though, just possible. 

Now that would be a great piece of equipment. Like a battery load tester. Battery measuring 12.5 volts but fails immediately under load.
I found a picture of a Yamaha Electro-tester. It says it used a spark gap of .25" as a measure of a good ignition system...now that is a GAP.
3.bp.blogspot.com/-EpZKld6dsU4/Tgtv2vPME...1600/Picture+010.jpg

As a matter of information, can you elaborate on the mating of a 'R' spark plug on our vintage machines?
It seems with this SC500 having a resistor spark plug cap, the old B8ES non resistor plug would be appropriate.
I have purchased B8EG spark plugs in the attempt to stay compliant and not 'over ohm' the system.
Am I correct in assuming this?
 

A few years ago I managed to bag myself a new one of these (for a lot less than this seller is asking!).

Yamaha Pocket Tester 90890-03112 | eBay

Apparently the British army bought a load of quad bikes from Yamaha and these pocket testers came with the back up spares, and the seller I bought from had some army surplus.

From a different buyer, I bought a box of slightly used thread taps for next to nothing. He was an engineer working on a contract for the Royal Navy. The Navy/MoD buy all the tools that the engineers need, but the engineers get to keep them at the end of the contract, and this chap was selling off what he had been given.

I've got one of those Yamaha testers like Mark describes.
Had it at work since the "70's or maybe late "60's.
Initially with the single wire magneto AC ignition coils we thought almost every coil was faulty but Yamaha put us right by telling us that if they draw less than half an amp then to test them on 12 volt.
Back then most magneto ignition systems were 6 volt on the DC charging side to the battery so we tested the single wire AC coil on 6 volts.
Often tested ok but the odd one drew less than half an amp so 12 volts livened them up.
Too long ago to remember but maybe the "less than half an amp" coils were shorting out inside giving less resistance hence less draw?? or would it draw more?? & faulty--not sure now.
I guess a bit lol. Never had many truly faulty coils over the years.
It was good with the adjustable gap in a glass window & was interesting how far the gap could be widened until it misfired.
It was the tester that tested "everything", not that it ever got put through all its modes.
Talking the Yamaha multi meter like in a post up top, mine got damaged when i forgot to turn the mains power off while testing my hot water cylinder thermostat.
240volts AC here in nz.
It went poof, & smoke haha. Had to go change my undies after that.

Not mine but identical & doesn't look as abused as mine.
photos.app.goo.gl/nQvfS81eLmF6ZSW16