30km/l is EXTREMELY low for a 70cc.
Even my 2-Stroke Yamaha returns ~40km/l easily when ridden moderately! And my old 1992 DR350 (350cc single-cylinder) used to return 70 MPG = ~29km/l.
No type of engine problem could possibly cause your 70cc bike to return fuel average comparable to a 350cc motorcycle, unless there's a hole in your fuel tank!
Try to change your petrol station. I'm sure you're not getting the amount of petrol you're paying for. Most fuel pumps are infested with dishonest workers, sadly. And if that's not enough; they also sell poor quality, expired fuels to maximize their profits; which also greatly affect your fuel economy.
Try to get the fuel tank refilled from a multinational brand, such as Shell, which I personally use; and recalculate your fuel average.
Alternatively; your odometer might be faulty, which is usually the case with most Chinese made motorcycles. Calculate your route with The GPS for maximum accuracy.
