a pipe behind the intake was leaking in my case (pipe was cracked badly), the mechanic poured some water from behind the filter area, and it sucked the water in to the engine, he diagnosed the problem this way, I have got the pipe from junkyard worth Rs. 450/- installed it and the problem solved, never faced that problem again, car became smooth, and air sucking noise like sooooooooooooo was also gone, feeling the difference, another issue might be with advancer settings, first you have to tune up the car on petrol with correct ignition timing, then set the advancer, => procedure of advancer setting is mentioned here https://www.pakwheels.com/forums/cng-lpg/171009-installation-functions-advancer-emulator