Alnarv,

I completely agree.  When you were talking about the "collapsing magnetic field" I was thinking you were speaking about the field in the IGNITION coil, not the source coil.  I reread your post and see you were talking about the source coil. 

I'm so used to pointing out the ignition coil on these systems generates a spark when the magnetic field is generated rather than like it does when it is collapsing (battery ignition) that I missed what you were actually saying.

You are absolutely correct that the source coil "voltage spike" when the points open is much higher than the AC sine wave voltage peak.  The visual I use (while not literally the same thing) is somewhat like having a water hose turned on low...  water is trickling out of the hose (similar to low voltage with points open)...  now block the hose with your thumb (similar to points closed)...  wait..  now remove your thumb (points open again) and the water surges out of the hose and squirts much farther than the trickle did. 

It is this "surge" of voltage that hits the primary winding of the ignition coil and generates the spark.

I may have found a way to mount the dial indicator base. An articulated arm mount that is very old. I've got it soaking in an oil bath in the heated ultra sound cleaner to see if I can get it to free up again. I'll be able to mount the magnetic base to the frame of the bike and hold the DI over the piston.
If this works I'll set the points using the dial indicator to the proper setting and see if there will be a spark.

Exactly! I use a similar "mental picture". Now for Rapidrob, the troubleshooting part is to determine whether he has that "spike" of sufficient voltage to trigger the ignition coil to do its thing. That is where using a scope or fast update on his Fluke can point to where to go next. If he has the spike, no need to look any further into the mag assy. If not, the problem is there. With .015in point gap, I can't believe the timing can be off enough to prevent spark. Since he has proved the other components to be O.K, the source coil is the only likely candidate left. (assuming he does not detect that "spike")

It took four hours to repair the articulated arm to hold the Dial Indicator. It was given to me and had not been used for many decades and was rusted together and the center steel cable had stretched.
It looks as though it will work to set the timing properly.
i.imgur.com/6a5QOLa.jpg
With the head off, it should work.
i.imgur.com/AQrLyxL.jpg

i.imgur.com/SNDPu29.jpg
 This what I use screws on the cylinder stud also are you using good quality points not cheap Chinese points 

Alnarv,

There also have been several reports of brand new ignition coils that might have good "resistance" readings but simply won't spark.  There doesn't seem to be any interest in checking for the "spike" even though the tools are in hand or making sure ignition coil is good by testing it on another bike or swapping to a "known good" coil.  So it's time to sit back and  

10/4!

I will check for the "spike" with my O-scope after I verify that the points are set correctly.
I do not own another bike with an ignition coil like the Yamaha.

Rapidrob might be the one to ask what he thinks of the yamaha points system on those early magneto AT CT DT where they earthed the points ignition source to within the lighting coil.
Someone once said it's to boost the spark at high revs.
Ignore the rest if you like as not relevant to the old system.

It just came to mind after i found a couple of old cdi parts stators from my 465 & 490 days long ago where i'd left the high speed coil off & gone for a simple system after having problems & just used a typical simple Yamaha scooter cdi box which i had.

Sold the 490 long ago but it ran faultless for many years.
Was i missing out on top end horses or was it just less likely to foul a plug--which it never did anyway.
Or is it more likely to be to do with varying the timing at revs, hence calling it the high speed coil.
The high speed cdi coil only has about 20 ohms--to add to the 350 ohms of the main source coil & meets up in the original cdi box via a red wire into the box.
The main source coil wire is brown.
The scooter box i used just had a black-red which is scooter source coil color.
I put the scooter box black-red to the stator source brown plus the pulser white-red of course. There was also a black going out of the scooter box to the kill switch just to ground the source i guess--or pulser.
It worked anyway.
Thinking back 20 years it's possible my high speed coil was faulty--or the box was--so probably why i took the easy option seeing i had a simple box that worked with one source coil.
Sorry about the essay & feel free to bypass it as i sort of wondered off the case a bit haha.

I'm not impressed with the magneto system at all. In the 60 plus years I've been playing with engines, never have I seen one where you have to pull the head off the engine to set the timing!
And then guess is the Source Coil is putting out the correct voltage as a "spike" and hope your test equipment is fast enough to catch the 150-200 VAC spike. (Vids show 15 VAC )
We wont talk about the tiny slots in the flywheel provided to verify the points are starting to open to set the air gap on the points.
Other than that, what a great engine to troubleshoot.
I will get it to to spark one-of-these-days. It's now a challenge.