working on a '72 250 (dt) that has been butchered in every way possible.  Where I'm at now is that it runs with the key off and stops with the key on.

It should be as simple as the wire from the rectifier going to the coil?

The Japanese wiring diagrams are a joke, and I dont know if the wires are the correct color or if they have been changed,

So before i go and pull all the wires and start over,  do you have any guidance?

thanks
Rich 

Just reading an electrical schematic is tough.. period.
But the Japanese schematics for these old bikes are good in my book. It’s helped me with a RT2 (1972].
Rectifier is used to change AC voltage to DC for the battery side.
Your switch must have some wires hooked up wrong. GR8 Uncle AL has the schematic written out somewhere in the depths of this forum. I’d suggest doing a search. Do spend time studying the wire schematics.. a magnifier will help. Follow each wire. The worst you’ll do is be cross eyed for a few hours.
Sneezles61

It has a magneto ignition.  There is a wire from the points to the ignition coil.  (highlighted in red below, wire should be black with white stripe but is sometimes just black). 

Ignition is shut off by a wire that is teed into the wire between the points and coil.  That wire goes to ground.  When the switch contact are closed, the connection is made to ground and spark is killed.  When the switch contacts are open, the connection to ground is broken and you have spark.

Someone could have installed a different switch that is for a "battery ignition" bike?..  On a battery ignition switch, the contacts close when key is on to power the ignition.  On your magneto bike, the contacts open when the key is on to stop grounding out the points wire. 

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"GR8 Uncle AL has the schematic written out somewhere in the depths of this forum. I’d suggest doing a search."


Here we go - post #2.

72 DT2 key switch wire diagram - Vintage Enduro Discussions (yamaha-enduros.com)