Another update.
Rode the bike on more open trails allowing 4th, 5th gear and the fuel consumption was better than last time (see above).
I kept the carb set up. Just opened the air screw a tad (less than 1/4 turn) and raised the idle slightly as the temperatures were pretty high (85F).
Bike used 8,34 liters for 100kms. I am happy with that, so the problem seems to be solved. From almost 13 liters to about 8 is a significant reduction. I am happy now
