Curious how you determined where to make your 3.2mm mark? 

While the magneto has a "timing tab" and corresponding mark on the flywheel, Yamaha never even mentioned an "align the timing marks" method for static setting of timing.  In my experiments over the years, I suspect that's because the accuracy of setting points using static timing marks is plus or minus 5 degrees or so.  (Your timing tab is badly bent in your pictures, but it could be fixed to use as a reference which I'd recommend doing)

A very well respected two stroke tuner in the US told me several years ago that modern fuels have a much faster burn rate and highly recommended retarding the timing slightly for best performance and to reduce detonation.  I have done this on several bikes and found that the larger bore bikes do seem to run better and kickback is greatly reduced.  So you might try retarding the timing to 2.8 or even 2.5mm BTDC.

On your shifting issue, most likely cause is a missing shift detent ball and spring.  Many times the retaining bolt is mistaken for the transmission drain plug.  The ball and spring falls out and gets thrown away with the used oil.

Ok. What do you mean with „timing tab“ ? 
where is it? 
I dint see any markings on the rotor so I did my own  
By setting the piston to TDC and marked it somewhere. The red marked point on my pictures. I did this by applying a dial gauge on top of a tool which I can put into the spark plug hole.
Then i took back the piston about 3.2 mm and took the second mark for the ignition on the flywheel. Exactly at this point when ignition marking and my red point match together then the points should open and also later on by checking with timing strobe. 

And mark can you be more in detail of that. I did an oil change last year maybe I lost that oh no!

Hi Mark. Do you mean #9 and #10 on that page ?
www.cmsnl.com/yamaha-dt250-1978-usa_mode.../partslist/C-10.html

#10 and #11

Yes 10 & 11 sorry!
what is the purpose of that parts? Thanks 

Shift detent.  Spring-loaded ball rides on "star cam" on shift drum and helps locate next gear and hold bike into gear when shifting.

Pictures showing stock timing mark on flywheel and timing tab on stator.  Your timing tab is bent over to the right.  Edge of tab should line up with mark on flywheel when piston is 3.2 mm BTDC




And again, there can easily be an error of a few degrees using marks and Yamaha did not use conservative timing settings. Plus fuels have changed a LOT in the past several decades.  I strongly advise simply retarding the timing a few degrees and see what happens. 

Another thing is there is often a few degrees of slop in the keyway which is why Yamaha recommends re-timing every time the flywheel has been removed...  and it also adds to the inaccuracy of using timing marks. 

I know it's often possible to ballpark the timing and everything is fine...  I've done it many times...  but I also know there is a reason Yamaha used such a precise method.  Timing errors can stack up in the same direction and just a few degrees too much advance can easily make the difference between kickback and no kickback.