Hello i just picked up a dt250 1974 i'm having issues and it's driving me nuts.
the bike fires up if the headlight is plugged in and light switch is on the ON position and the High beam on.

If the bikes running and i either turn the headlights to the off position it dies instantly same for it if is running and i turn low beam on it will die

with the headlight unplugged it gets no spark period 

So far I have tried new lighting switches thinking it was grounding out and killing the bike
both new switches do the same thing
i removed the lighting coil after seeing it was damaged thinking maybe that was grounding something out but still no change.

any help would be appreciated .

also should the black and yellow wire have continuity between them off the coil ?

DT250 is mag ignition unless changed to later cdi. So black from the mag is your only wire to provide spark. So run 'that' mag black straight to the plug coil & see if she keep running.
Re-reading, you now have the lighting coil 'out'/ Not sure what to say next--make sure the ignition stator coil is earthed. Earlier DT's with two lighting/charging coils had the lighting earthed into the ignition source somehow--not sure how it worked. But if one lighting was loose on its post & shorting out it played up in the ignition. Don't think yours will be like that. Original issue sounds a bit like wiring colors mixed up. There's different shades of browns & greens although that's not ignition related. Bikes eh!! just when i thought i knew it all--i no nothing. Always new problems. Points gap within spec & not way over 15thou!!.

Engine mount bolts tight. Loom earth ok at the coil mount under the tank. Rectifier not blown & backfeeding the battery. Should run with ign switch totally unplugged as only earths out the wire from the mag to stop it Black with white tracer would be involved with that.

Greetings from Tennessee  

Umm, greetings from me too in NZ.

You are going to need to disconnect the ignition switch completely.

Run a 6 volt power lead to the OPEN POINTS ( cardboard ) and touch the red 6 volt wire to the condenser with the spark plug out and connected to test the source coil. You should get one spark per.

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I will try this Friday 
tha KS for the reply everyone. It does run with the ignition unplugged I did try that last night. 
I'm gonna upload of nice video of the bike running and dying soon as the switch it moved 

You could have a bad ignition switch. Put a tooth pick or a stick in there with the key still in on position number 1 where it stalls out - wiggle the key..

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