Your CNG consumption is high i think it should be giving you way 100km per cylinder in city. Atuned CNG kit should give you good pick with economy. Generaly timing for cng cars without advancer is 20 degrees give or take a few. Tappit are set on a gap of 25, roughly, plugs on 70-80. The reason for delayed shut down on petrol is probably due to wrong timing or wrong tappit gap.
For Cng optimum consumption if tuning doesn't work get your kit serviced