Andreas,
If the 19c reading is only between 2000 and 4000rpm, that would probably be a problem with needle jet and jet needle. They control the midrange fuel mix. I would remove the needle jet and jet needle for cleaning and inspection. The needle should be smooth with no wear where it fits into the jet. The hole in the jet should be clean and perfectly round. Sometimes they will be worn by the needle. Check to see that both needle clips are in the same slot. 19c is really not bad but you should be able to get a little closer. It is usually the jet that is worn and not the needle. If you suspect the needle jet, switch it to the other carb and see if the temps switch on the gauges.
Bill.