Later 400 is a better setup but would need the cdi box as well.
Very last 250 had the good cdi same 400 mag too.
I must say though that your problem i one i've never come across.

Here is a screen capture I made from a video a few years ago of a 75 or 76 400 with the exact same issue.  "P" is the approximate location of the timing mark on the pulser.  You can see the strobe froze the flywheel mark (F) considerably advanced from where it should be firing. 



If I remember correctly this bike was running fine and then started kicking back bad one day. 

Later model 400's have had the same issue. 

That's clever photography Mark. Did you find it'd broken the crank key haha. Ok not that because it'd still show lining up right except the crank/piston would be in the wrong place with the key broken. Or am i wrong. Strange things these early yamaha CDI's. I'll stop overthinking it.

Yes Mark that pic looks very familiar - that's where mine is firing. Very odd indeed! I wonder if that's the same bike I now have

Well you guys saved me a lot of trouble trying to fit the RT3 points system to this 400 - thanks! Also saved me time in trying to figure out what's going on with this system to begin with, my experience with other CDI systems is they either work or they don't, not this random "let's fire here" nonsense. Probably a failed 3 cent resistor on the CDI board or something trivial to fix, if you only knew the problem... I suppose I'll bite the bullet and just get a Vape system with all the 12V bulbs/etc to go with it. Will end up saving me time and frustration vs trying to go another route. I just hope the new owner sees the Vape as value-add.

There is another option made in England.  I have never used it and probably won't because of the early history and some of the "lack of customer service" people got who had problems and weren't in the UK and didn't have the luxury of returning it in person.
Several people have posted about their latest versions and they seem to work much better... I can't remember any recent issues.  I think it's a lot cheaper than VAPE too. 

I'm not sure but I think you might have the option of retaining your 6v charging system or going to 12v. 

Yes I looked into it - Rex's Speed Shop in England, they have an "ignition only" kit:
rexs-speedshop.com/product/dt360-dt400-b...nly-kit-no-lighting/

I looked into their instructions for grafting the stock 6V lighting coil(s) onto their stator plate and I have a feeling I know how that will end up:
- I'll attempt that and spend 4 hours on it - I could have been spending that time riding a bike that actually runs, like my DT3 or RT3
- I'll screw something up because soldering isn't a strength of mine
- I'll wish I'd have just bought the full 12V kit with lighting coil
- I'll cuss a lot
- I'll try and order their 12V lighting coils so I can now fight adding them to the "ignition only" kit
- They'll tell me all the reasons why I should have bought the whole kit to begin with, and make me feel inadequate and simple
- I'll cuss a lot over paying $40 to ship a $40 winding
- It'll still never be right
- I'll now have the same $ into this budget system as I would have into a Vape and it still won't be right
- I'll cuss a lot wishing I'd have just bought the Vape to begin with
- I'll say screw it, throw the English stuff in the bin and buy the Vape anyways

I've done this sort of thing a while now, so I'm skipping to the last step and telling myself this is the benefit of Wisdom I'm leveraging. I'll make peace with the situation and all will be right with the world and the DT will be better for it. That's the plan anyways. We'll see how the actual execution goes.