Try to change your air filter and dip it into oil. When i bought my gs150, i did not check air filter for more than 4 months and i was getting 46 to 50kml. Last week i checked air filter that was broke so air was going freely with all those dirts.
I changed the air filter and dipped it into oil. Now my average is 38 to 40. Thats because more fuel is burning than air. At least my gs is getting clean air.