I wish I had a manual that would maybe show the operation better.  I suppose Yamaha could could have used two pick-up coils or something but I've never seen that done. 
 

I will try the fix in the video , if it dont go well ive lost nothing . Will keep ya posted how it goes.

I found some info on a UK XT250 from that era...  diagrams show the basic two-wire source coil and two-wire pickup coil so nothing that appears it has a separate sensor for the advance.  www.scribd.com/doc/95729057/Repair-XT250-Yamaha-1980-1983

On the baking, use caution on the temp...  especially he is using Farenheit and not Celsius/Centigrade scale.  There are also many other methods...  I think my friend mentioned starting with a lower temp like 200F (93C) for a longer time like an hour or two.  And if that doesn't work he raises temp and shortens time. 

Replying to a couple of pages back yes advancing is all magic in the box. It'd be real good if you can borrow a known good one to plug in. Only reason I say that is. When they blow & stay on retard you'd be struggling to get the revs past about 2000 & would just hold back & maybe spit back through the carb & try to blow the carb out of the manifold. I'm just thinking back to the few I've had at work a lifetime ago. I say all that because you said you can get revs but maybe I'm misunderstanding a little. Pulser coil needs about 30 thou feeler blade under it but as I recal experimenting once if the air gap is closed down its just slower to get to advance but still goes there. Hope I'm not making a pigs ear out of explaining things. So all cdi & stator plugs are original not substituted fir individual plugs? So can't go wrong connecting things. I know what I'd do but maybe better not. On one I moved the flywheel about 30 degrees & fitted it minus the key. Did it up super tight to prove a point and apart from expected kick back she ran perfect. So point proven--box not advancing. Haven't proof read so hope it's legible.

revs ok when im testing on the bike table but when taken out on the road & try and open it up it coughs & splutters , does pop a bit & is dog slow , i will test the gap between the pulser coil & flywheel & see what that shows . as far as im aware stator, CDI etc are all original .

Way back did you say the source coil had been rewound. Not sure I'm reading anything into that except I've got a suspicious nature lol. Like why was it rewound, no spark at all or chasing the fault were still chasing etc.

No it was not rewound but the wires were shot so i had it rewired & a new loom to match from REXs Speedshop

imgur.com/P6K6UsP

Ok, so the wires to the sourcy thing were rewired.
I'm sounding like a broken record i know.
So red is red & brown is brown?.
They come off each end of the ignition source coil.
Bike looks nice i must say.
Lets get it running nice. & you've mastered posting photos, Cool.
Just another random--i hate that word the grandies use--.
Is there a chance the source coil could be fitted flipped over end for end? if it was taken off for it to be rewired--not rewound. Mostly they cant be flipped but without a pic in front of me i'm guessing & flying blind.

Checked the source coil gap all ok then put the CDI in my parts oven for a roasting ,what i noticed was lots of little bubbles on the epoxy part as it warmed up ,put back on bike & i am amazed when revved it advanced towards the source coil , still got to road test it once all back together but so far so far so good. A very big thanks for all your help, will let you know how it goes. Regards