This issue of yours can be as minor as choked fuel filter or inefficient fuel pump OR
it can be (most probably) blocked carburetor fuel lines and vacuum ports.
Get the system checked. Service the carburetor thoroughly from inside out with proper calibration of float and needle valve.
Set IG timing (without vacuum advance) to 15 degrees BTDC at 900RPM to avoid missing (Dieseling) on petrol without considerable loss of power on CNG.
Fuel Filter: 50/-
Stock mechanical Fuel Pump (Japanese): 2000/-
Carburetor service Labour: 500-1000/-