how much oil was already there in your engine when u changed your oil ? i mean how much less ?
whats the CNG pressure from where u get ur tank filled ?
since when last time u got ur CNG kit cleaned ?
Is the air inlet properly choked ?
D series engine were one of the most economical engine made by HONDA , there is definetly a problem either with ur engine or ur CNG kit - otherwise locally u should get around 120 km / tank