Congratulations on your road trip.

Normally this is caused by a lean spot in the carburetor. You will need to narrow down the cause.

Often the cause is the accelerator pump not putting out the full shot of gasoline. You can test this very easily by doing the "Accelerator Pump Test" outlined in Carburetor Adjustment.

Fuel economy of 14 km/l is very good if you did a lot of stop-and-go driving or going up and down hills. On a level road B310 can get around 16 km/l at 100 kph.