Just commenting on this even though it's old in case someone is googling for fixes. I'm new to working on bikes, so have just learned by google. But, had issue with my son's 2003 XR70R not starting. First issue was no spark. Don't know how to do the Vmeter tests, so I just started replacing things b/c they're pretty cheap on ebay. First spark plug, then ignition coil, then CDI, then Stator. Of course, it was the stator that was bad. Got it running, and it stopped running shortly thereafter...would not start again. Looked at stator, and wires had burnt. Not sure if it was b/c it was a cheap one off of ebay, or what. But, put wires back together with some electrical connectors, and had spark again. Started it up, drove it for 3 minutes, and it died. But, still had spark. Cleaned carb, tried new carb, nothing...would not start. Sprayed starter fluid, and still wouldn't even kick-over for a second.
I was completely dumbfounded, as I had checked valves, timing, etc. previously while I was fixing the stator wires, and all were correct. Finally, I took the stator cover off, and apparently, even though I had originally ensured the fly-wheel was installed with the key/notch aligned, what I didn't do was tighten the fly-wheel on properly. The fly-wheel was off the notch, so timing was off 180 degrees. Took it off, used a marker to mark the notch so I was sure it was aligned. Tamped the fly-wheel secure with a hammer this time. And, really bolted it on tight this time. Started right up, and has ever since.
So, if you're trying to diagnose, you're sure it's not the carb, your sure it's not anything electrical, and you're sure it's not the flywheel mis-aligned b/c you put it on correctly the first time, give it another look.