i would second the comments by "iLLDuDe786" and "margallar".
Definately your distributor is advanced too much to give a low end pickup on CNG.........may be good for CNG driving but bad for petrol.....
Note....On CNG your carbi engine timing should be 12 Deg......and on Petrol it should be 8 Deg......I am assuming your cultus doesnt have a STAP 00 (timing advancer for carbi) installed........so when you switch to petrol, the ignition timing remains high and causes knocking........
You have 02 options, either get your timing reduced by a couple of degrees (lets say 02-03). It wouldnt hurt much pick on CNG and would eliminate knocking on petrol.....
Option 02: If you frequently use petrol in your CNG equipped Car....i would recommend a timing advancer (the one that actually lags timing on petrol by almost 05 degs..but we actually call it advancer). This would keep the advanced timing on CNG and reduce it on petrol.....
You can also give a try to high octane.......but i would say its not a permanent and suitable solution....