I measured 4% roughly on the rear cylinder and 13-14 on the front cylinder.
I have synched the cards however maybe I should try it again.

It must have been condensation in the cylinder, I tried again to hold my hand in front of the front cylinder exhaust and the result was somehow different. I don't know if it is the smell of burned oil or just result of running rich.

I have recorded two short film clips yesterday of the bike.

One where i start it up cold in the morning: (The sound on the video is very 'mechanic/metallic' that is where is back fires.)
https://youtube.com/shorts/vzHNE871RM4

and one where it has been running for 20 km:
https://youtube.com/shorts/TG7u3A7eCRU

I am considering that it is a broken valve stem seal or something wrong with the valves on the front cylinder.

Good Morning Kennihs,
to your starting Audio: It sounds that your ignition is not working correclly. First impession was "only one Cylinder" is working.
That is corresponding with the black face of spark plug Cyl2
Q1: Do you have a Flash-Light for checking the Ingnition Point? If yes, check both Cyl 1 and 2 for proper timeing.
Q1.1: Does the Spark Plug Cap(s) is/are in good condition?
Q1.2: Does the Ignition wire Cyl2 is good at the end which is drilled in the Spark Plug Cap. If not, cut a little piece off to get a good end before
        connect it to the spark plug
Q1.3: Ignition Coils: Cyl2, is the left one (seen in drive direction) Check electric wires for good conditions. Are all wires without any cracks,
        are the connection plugs without corrosion on the contact surface?
Q1.4: Check wires from the pick-up coils. (left side of the engine, right abow the clutch-link-cover, in front of the air-filter-housing. See Pic.
        Check for thermal deformation because of the head in the nabourhood of the Cyl1 exhaust.
Q2: Double Check the Starter Plunger, they are Rearly fully closed. Check it by push them down by using your finger.
Q3: All O-Rings new? You can check it, while the engine is running, spay it with brake cleaner a little bit.
     If rpm rises with the influence of alcohol the O-Rings are not good.
Q4: Pilot Screw (s): 2 evolutions out from full closed position?
Q5: JET-NEEDLE: Cyl 1: Y-21  and Cyl 2: Y-20 , The Cyl1 (rear) is tendencly more rich then the Cyl2 (front) for inner cooling.

If you are not sure, whether the spark plugs are ok, if they are new or not, innerchange them form cylinder to cylinder. If spark plug from Cyl2 is working correctlly in Cyl 1 it is ok     a.s.o.

Syncronizing the Carbs:
1) Be sure both Throttle Flaps ar not closed. So you can use the Throttle cable at your handlebar. Therefore use the Cable Aduster.
1) You are able to sync the carbs properly or must you turn in the adjustment screw fully to there end in?
    If it is so, try to shorten the spring behind the adjustment screw a little bit to get the screw more in.
2) Do you have in place the screws for the Throttle Flap adjustment? See pic. Sometimes it is neccessary to use both of them.
   The right hand one is to understand as master to adjust Idle rpm during normal operation.
   During adjustment of the carbs is could be possilbe to adjust the left hand one, also. However, the left hand screw is to handle with care !
   Use it for fine adjustment after the main work of syncronisaton of the carbs is done. Only for adjusting Idle running.

With kind regards
Anja