I have already benefited from many useful hints and tips from this forum, many thanks to all of you who are committed and happy to share your knowledge!

Here are three photos of my dt250a:

Pictures (Dropbox)

I am aware that the bike is not 100% original, but that that wasn't my intention. I simply don't have the money for that ;-) The correct rear turn signal brackets are missing, the frame number and the engine number do not match. The cover under the oil tank is missing and there are still some small details that are not quite true to the original. In the end, I was able to complete everything electrically with help from my family. A friend of mine who lives in the USA brought me an extra cable when he visited us.

The dt250 runs when I kick it, but not yet smoothly. I'm assuming that the mixture isn't right yet, so I'll have to take a closer look at the carburetor. I hope it's nothing major. However, I will have a professional mechanic do this check. When I start it, the dt250 doesn't run smoothly yet. I'm assuming that the mixture isn't right yet, so I'll have to take a closer look at the carburetor. I hope it's nothing major. However, I will have a professional mechanic do this check so that everything can be checked again before I can get the motorcycle on the road.
 

Looks good! You probably want to install a voltage regulator to prevent blown headlight and instrument bulbs. Below is the TSB.

yamaha-enduros.com/index.php/service-bul...e-voltage-regulators

 

Looks nice. If it continues to not run well it would be helpful to do video so we can hear what's going on. Not easy to do a video i know.
If you have an old school mechanic who is familiar with older twostrokes then that's good.
Modern mechanics go more for pushing buttons to change settings via a laptop, ok maybe not that bad but!!.
Blocked up exhaust is often overlooked & in your case the tail muffler & deep down in that tail bit is a spark arrestor which traps carbon, & its all down hill for performance after that.