No spark-most likely cause-(besides the obvious)

[zoommx]

I was checking my jetting the other day after some motor work. After running it for 20-30 minutes the bike died and would not start, no spark from a good looking plug that was removed from motor or from a new plug from the box. I tried disconnecting the kill switch too of course. After I got it home it fires right up. I figure something is weak and when hot it failed. Which is most likely? Stator coils, CDI or ignition coil? Its my 78 yz100. I'll be breaking out the DMM this weekend to check values too.

 

[mideastrider]

I'll be breaking out the DMM this weekend to check values too.

That should expose the problem quick enough. Your lucky it will run awhile. Check everything cold then run it til it quit's quickly check again an it should show up the problem. I had a flywheel pick-up on a cr85 did the exact same thing.

 

[zoommx]

I checked the resistances and the only one out of spec was the charge coil 1. Went ahead and started it up and ran it hard for about 3 minutes. It started cutting out, so I shut it down. Wouldn't re-start, so I started measuring again, the charge coil 2 had an open measurement. Changed it out and it ran fine. Unfortunately as it is so often with these old bikes, I found another problem when I was replacing that part. The flywheel had about 1-2mm of movement, so there is a bad crank bearing. Ended up installing my spare bottom end until I have time to rebuild that.
THIS WEEKENDs RACE IT WILL ROCK!!!